ctrl-alt-postal said:
Shouldn't matter for Fallout, but almost every old game will have problems with multiple cores. So if you play a lot of old games(anything from before 2004 to be safe IMO), it's smart to get that optimizer working simply because it will fix(and prevent) a lot of errors.
On-topic: I don't really care about the bugs in Bethesda games, my problem with them is that their games are extremely poorly coded. In Oblivion it's possible to quicksave and then after quickloading a few times you'll notice new spells you never had while you possibly also lost a few spells. Something I've noticed in both Oblivion and Fallout3, there are locations where saves get corrupted and they're in a way tied to mods. In F3 I was unable to leave Megaton without the game crashing when I tried to save, this stopped after disabling the official DLC and in Oblivion mods only increased the chance of corrupting your save if you saved over an existing file.