A: It's complicated, and it has a lot to do with the nuts-and-bolts of warhead design.
The Reliable Replacement Warhead is understandably controversial. But I've been a little perturbed by the degree to which the forces behind it have been misunderstood in the public debate. The most common accusation is that the RRW is merely an excuse for the military to acquire "new toys," an excuse to develop qualitatively new forms of nuclear weapons such as bunker-busters, and pork for the weapons labs. A good example of this interpretation is this recent TAPPED post about the RRW.
It would be really nice if things were this simple- then we could just condemn the RRW program and go home. Unfortunately, the issue is considerably more nuanced, as is evidenced by the fact that when asked to review the RRW program, the American Association for the Advancement of Science came to the conclusion that it merited further consideration.
A great deal of ink has been spilled about the long-term viability of our existing nuclear weapons, all but a handful of which are more than 15 years old. (For some reason, the fact that W88 production has been going on at very low levels in the past few years isn't very widely known.) The primary argument was about the shelf life of the plutonium pits in the weapons. It was argued that these could become unreliable after they had aged for a few decades, which was not unreasonable given the fact that metallic plutonium has a number of really odd physical properties. Repeated studies, however, have found that the pits will remain usable for decades to come. Opponents of the RRW have used these findings to argue that "there's no need for new nuclear weapons."
This would be true if the pit was the only part of a nuclear warhead. Unfortunately, there's a lot more to a nuclear weapon than just the pit, and design choices made back in the 1970s and 1980s make stockpile stewardship prohibitively difficult. American nuclear warheads were designed with extremely tight design margins, and as a result use a variety of exotic materials that were chosen because of their optimal physical properties, while disregarding questions of expense and toxicity. Unlike the plutonium pit, these materials are known not to have multi-decade lifetimes, although most of the details are so highly classified that no-one outside of the weapons complex can evaluate the issues involved.
The weapons were also designed without any regard to the problems posed by stockpiling them for decades. Prior to the end of the Cold War, American nuclear weapons were generally scrapped once replacement warheads built to more modern designs became available. It was therefore unnecessary to consider the prospect of completely dismantling and refurbishing the warhead not once, but many times over a 50+ year lifespan. Combine this headache with the fact that some of the materials inside the warhead are extremely hazardous, and the nuts-and-bolts practicalities of maintaining warheads like the W76 become increasingly problematic. Although most details are classified, this article about Y-12's struggle with maintaining these weapons illustrates the problems involved.
Another strike against the existing warheads is the fact that they lack some safety features that they really should have included from the get-go. The most famous example of these is the W88's lack of a fire-resistant pit, which was criticized in Congress even before the end of the Cold War. This oversight (which was the result of prioritizing performance over safety in the design process) has resulted in weapons that place our SLBM subs- and submariners- at unnecessary risk. I suspect that one of the forces behind the RRW is the concern of the top naval brass over this problem, which could potentially cause a horrific accident.
The RRW is meant to address all of these concerns. It is designed to be maintained for decades on end, while eschewing the use of unnecessarily hazardous and expensive materials (where possible.) It is to include additional safety features, including a fire-resistant pit, and potentially advanced anti-tampering measures that will prevent unauthorized detonation of the weapon under any circumstances. The military also argues that the RRW will allow them to draw down their levels of operationally deployed warheads due to the increased reliability of each one. However absurd this may appear on its face, given the internal logic of American nuclear arms management this is actually pretty reasonable- as is attested to by the AAAS report.
I personally believe that the RRW, or something like it, is an ultimate inevitability. It may be delayed another decade or so, but nuclear weapons aren't going away, and the Navy simply isn't going to tolerate toting around unsafe 50+ year-old W76 warheads on their state-of-the art submarines and missiles. I suspect that eventually RRW proponents will advertise the pertinent fact that the Russians have never suspended nuclear weapons production, and use the example of the Russians' new missile subs and ICBMs to argue for American investment new forms of these weapons. Given the decaying state of US-Russian relations, I think that many within the government will find this a convincing argument, leading to a new generation of US nuclear weapons and delivery systems.