Only reason to want more performance is when dealing with multi-monitor environments (EyeFinity - so, yes!) where more VRAM is also important... or to provide futureproofing.
Given the wave of console ports, I wouldn't worry too much about futureproofing until the new consoles (XBox "3" and PS4 or whatever) are out. ;)
Oh, I forgot benchmarks and bragging rights. Two very important factors. ;)
I don't agree
I play at 1680x1050
I'm happy to have my GTX285 for Titan Quest with Xmax mod (which increase number of monsters up to x10) (a G92 would struggle a lot more)
I'm happy to have it for Sacred 2 All options On
for GTA 4 too
and there's others out there...
I'm very exigent or even quite picky with the framerates achieved and I prefer no quality compromise if that's possible, so it really depends on what one wants!