I think you missed Saige's point here, posthumous. Saige's argument is mainly based on what she thinks the reason for continued research and manufacture of nuclear weapons is.

Once you can destroy someone, can you deter them any more by being able to destroy them even more? Is overkill somehow important here?

Oh yeah, definitely. That's the point. Ugh. I'm not an expert (or even knowledgeable) on what things the US is doing, but I can definitely think of good reasons for continued research and testing. I'm sure there are good reasons to create different sized bombs with different impacts. Wouldn't it be nice to have a bomb that could blow up a city but not leave behind all that nasty black rain? The idea isn't that we need tons of bombs so that we can blow up Moscow twice. We keep making better ones so that we can more effectively blow Moscow up with only one. Also, we need to stay ahead of the game, and make sure that we discover newer more powerful weapons before anyone else does. It definitely wouldn't be good if some other country found an easy to way to actually perform a succesful preemptive strike against us, would it? It's silly to think that MAD will hold forever. Who knows what weird technology might come in to existence to imbalance it. We need to be the ones doing the imbalancing.

Anywho, this isn't a terribly great argument, but neither is Saige's.