There definately was a difference when you were looking at stuff like lens flares, fog, smoke, some lights, or very colorful textures.
Anyways, I can see a use for 64 bit color, with 32 bit color we have 256 shades of every color(RBG), which might not be enough if you have a shade, and some transparency involved.
I dont think it's as important as 32 vs 16 bit, but once the hardware is capable of doing it at acceptable speeds, as it is with 32 bit today, why not go for it?