At first glance, there's something to this argument. If the nuclear stockpile somehow deteriorated, it would be nice to have some scientists around who knew how to build some new bombs, especially if other nations still had bombs that worked.
However, the Department of Energy, which controls the nation's nuclear arsenal, runs a large and active "stockpile stewardship program," in which scientists continuously monitor and test the components of the weapons. The know-how, the hardware, and the physical capacity to build more bombs and warheads—these things are not going away.
So, a few common-sense questions:
Does deterrence really depend on the refinement of a nation's nuclear weapons or on its pure and simple possession of nukes, crude or fine? (The fact that Bush hasn't attacked North Korea suggests an answer to that question.) Will deploying a refined nuclear weapon—say, a low-yield earth-penetrator—deter a foe from even bothering to dig underground bunkers? Or will it spur him to dig deeper or to disguise the bunker better? (The few conventional bunker-busters used in Iraq did their jobs well. The problem was that the bunkers were empty when the bomb struck, if in fact they were bunkers to begin with.) Will deploying such weapons dissuade a foe from building his own nuclear arsenal—or encourage him to develop one as quickly as possible, on the theory that otherwise the United States, newly armed with more usable nuclear weapons, might threaten to lob a few his way?
Finally, is any American president really going to order the use of nuclear weapons under any circumstances, for any reason, except possibly where not merely the vital interests but the very survival of the nation is at stake? (And if survival is at stake, the refinement of the weapon used is likely to be a peripheral issue.) If we're not going to use these mini-nukes, if having them doesn't enhance deterrence, and if developing them may encourage currently abstaining nations to build nukes of their own—for protection, if not emulation—then what is the point of speeding down this road any farther?