[Some editing done here, mostly to clean up. I did add a few comments to flesh out the arguments. BTW, the Judiciary postponed the vote for Sotomayor a week, maybe for the Republicans to read her written addenda.]
Sometime this Fall, the Court will examine three cases [involving the Second Amendment] that already are being pushed as “appropriate” ones for the Court to use for a reexamination of the Clause, and the Slaughterhouse precedent.
-- Lyle Denniston
This sort of this is more of an interest for those who debate the nuances of Vulcan languages than something that many deem very important. Sure, a few people -- most importantly perhaps several now on the Supreme Court -- care that substantive due process was used to provide a means of broad rights against the states pursuant to the Fourteenth Amendment when the Slaughterhouse Cases cut the legs beneath the Privileges or Immunities Clause (not to be confused with the clearly related but different Privileges and Immunities Clause of Art. IV).
First, a bit of confusion from the discussion:
Under constitutional theory, there are only three ways that the Court could interpret the Second Amendment as applying to the states. The Constitution's text rules out one of those, the Slaughterhouse Cases rules out a second, and the one remaining — “incorporation” of the Second Amendment into the Fourteenth Amendment so that it reaches states – is not an attractive option to constitutional conservatives. Thus, the impending challenge to the Slaughterhouse precedent.
The discussion clarifies that the text of the Second Amendment only applies to federal laws per "the constitutional understanding" from at least Barron (1833). This is important -- the "text" alone does not say this. It is a sound interpretation, if not one everyone now or then accepted, but clearly not one where the text alone solves everything. After all, the Slaughterhouse majority insisted "the text" of the Fourteenth compelled their result (the clause covers only a limited number of rights, not the Bill of Rights). The discussion in effect appears to implying one is right ("the text"), the other at best open to question (citing a case).
Why is it helpful to overturn a hundred years of precedent and provide a deep substantive security to the Privilege or Immunities Clause? As noted by the amicus brief cited, largely the text and original understanding allegedly is better secured. There is a good argument here that the clause was originally thought (though as the dissents in Slaughterhouse suggest, even then there was some understanding of substantive due process) as the core protection of rights against the states such as those found in the Bill of Rights. But, especially since there is also an original understanding as applied to the due process clause, so what? Strict originalism, especially if it requires overturning a body of law mainly for neatness sake (see below), is not widely held doctrine. Is it really like a full Court will bite?
Six justices at least would be perfectly fine with the Due Process route. Justice Kennedy has shown no desire to sign on to Thomas' Privilege or Immunity proposals. Justice Scalia dissented in Troxel v. Granville, where Thomas suggested that that right would include (as it surely was intended to) something more than the rights written in the Bill of Rights, particularly parental rights in that case. Roberts seems to prefer narrow looking holdings when possible, not strictly concerned with doctrinal neatness to the extent of Thomas, though might be game in theory. Again, the problem with the original case is that it denied the amendment of much force ... like water seeking an outlet, the Court found another route. When did it find a need to re-frame a right, except perhaps some justices in respect to abortion (equal protection)?
It is suggested, as noted by LD, that it also is believed that the right in question would be less open-ended. I find this almost laughable. Several of those cited by the brief have a broad view of its terms indeed as shown by those who think it would be the proper route in cases like Lawrence v. Texas. In one case, Justice Thomas (who would apply it to things like care and raising of children, a privacy right that can be the basis of the right to choose an abortion) suggested it in a funding context as a limiting device. Saenz v. Roe. But as shown in the First Amendment context, you need not think funding per se is a right to suggest that it can be used in such a way to unconstitutionally infringe one. And, in the process of saying funding was not among the rights intended to be secured, Thomas cited open-ended discussions of what exactly are.
Also, "liberty interests" and the like is how generations of people in and out of the legal academy have understood things. It is quite true that "privileges or immunities" can be used to cover similar ground. But, just as those who ridicule substantive due process as a contradiction in terms laugh a bit too much*, it's a bit more complicated. Many consider "privileges" to be less than "rights." We give the former a somewhat different connotation. And, "immunities" in particular seems pretty technical and minor. As with "penumbra," maybe this is just petty lack of understanding, but things like that matter. In fact, the reference to the "privilege" against self-incrimination itself implies difference from other "rights," particularly ironic when the Fifth Amendment itself in no way says "privilege" as applied to this matter at all.
Likewise, "privileges of citizenship" appear to cover narrow ground. You can readily show how most of the First Amendment (religion is more tricky, but it can be seen as functional) fits in and with a bit of thought ditto most of the others in the Bill of Rights. The militia and jury are citizens with a special role, and Saint Paul in Acts claimed special due process rights as a citizen. But, the Ninth Amendment references an open-ended number of rights that do not seem to be connected to "citizenship" in particular. It is true that the rights of "the people" are referenced, but again, as with the Fourth Amendment use of the term, we tend to not really consider that too closely. And, we are left with protecting non-citizens indirectly via due process or equal protection.**
Why not use a more universal security like due process that covers "persons," one that has the weight of precedent, and some component of original understanding to boot?! A security the speaks of strong sounding "rights" and "liberties" (Give me privileges or immunities or death?). The "citizenship" component of the Fourteenth Amendment can be used to overrule Dred Scott, giving full breadth to citizenship causes of action in Article III, protect the (not totally trivial) privileges and immunities of national citizens (increasing federal power in the process via Section V in particular), and yes reinforce the other provisions in various ways. It will not really be a "nullity."
A narrative of rights of U.S. citizens can and has been made [citizenship itself has certain rights connected to it as citizenship of our own nation does in particular; see, e.g., the Second Amendment and a 'free state'] but so is one involving "liberty" growing out of freedom secured by the Thirteenth Amendment and beyond. [Let me add that there is something to doing more to focus on the former, including how citizenship in a "republic" implies certain rights and institutions. And duties, such as jury and militia service. But, this can be done within the "liberty" framework too.] A more universal narrative that the habeas cases like Rasul underline is in favor. One that protects all "persons" immediately, not as a sort of afterthought.
Line drawing will have to be done in both cases and some of the supporters here have a broad view of what privileges or immunities would cover. And, use of new terms and terminology (does "citizenship" change anything?) seems more trouble than its worth with so little real payback. It seems more an academic exercise than anything else.
Not that I disliked them -- let's see how it goes, hmm?
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* The amicus brief cites two Elys, one that ridicules, the other that supports substantive due process.
** Non-citizens as "persons" would be protected equally as citizens unless there was a significant reason to treat them separately with the additional concern that classes among races and nationalities is deemed particularly troubling without compelling justification. Then, they still would have some base set of rights. Comparably, blacks in some states did not have rights (or privileges and immunities pursuant to Art. IV) as citizens, but still some rights as "persons."
Also, perhaps the default rule is that everyone had rights generally applied to citizens (or "the people"), especially if they were long term legal residents of this country. As two justices suggest (Kennedy/Stevens):
Given the history of our Nation's concern over warrantless and unreasonable searches, explicit recognition of "the right of the people" to Fourth Amendment protection may be interpreted to underscore the importance of the right, rather than to restrict the category of persons who may assert it. ...
In my opinion aliens who are lawfully present in the United States are among those "people" who are entitled to the protection of the Bill of Rights, including the Fourth Amendment.
John Hart Ely Jr. made a comparable comment in respect to the provision in question, including (iirc) in Democracy and Distrust. He (and others) also suggested the Ninth Amendment would be a possible alternative route. Again, maybe ala Casey, we can build on old precedents, noting how many provisions in fact lock together to secure overall principles.