If they go to 64-bit they can take advantage of additional security features in Windows 8 like ASLR essentially for free. WebGL might also see a bit of a boost. And never underestimate the number of people complaining because they have a 64-bit OS but only a 32-bit browser, or even people who will think 32-bit software won't run on a 64-bit Windows. It's true it will offer few tangible benefits, but it's also just the game that has to be played for people who are quite happy, even proud in some instances, in their ignorance and will never listen no matter how many times/ways you try to explain to them there's little to no benefit.