Even without a Steam beta account you can try 3 demos to see why Steam would work best with 32 bit dynamically link games only (when you want to be able to use the steam overlay and not only start the game):

a) 32 bit apps, dynamically linked
b) 64 bit apps, dynamically linked
c) 32 bit apps, statically linked

There are the examples:

a) World of Goo, Amnesia via linux32 wrapper
b) Amnesia on 64 bit
c) Unity of Command (use as option: sdl-fullscreen for better fullscreen mode with kde)

Only for a) the overlay which is done via a LD_PRELOAD lib works, you would need to have steam+app from the same architecture to let this work always, but not all 3rd party apps are 64 bit. So it is logical that Steam is 32 bit. But compared to a real package management system Steam has a huge fault: it does not install required depends. Also when you have to work around libc6 bugs via an extracted ubuntu libc6 package in the ~/Steam/ubuntu12_32 directory this only works for type a) games. If you want to play c) on Debian you have to extract the package again in the bin dir of the game itself (~/Steam/SteamApps/common/Unity of Command Demo/bin). mc can do that very easyly.