Currently games take quiet a performance hit when using Unity (the default DE, not the engine). This will probably be improved/fixed in the next couple of releases, now that it's a priority for Canonical, but the fact remains that Ubuntu 12.04 will be around for quiet a while and some resources will allways be lost to the DE.
So it would be really cool if Steam could start without a distinct Desktop enviroment. This could be handled similar to XBMC: After you install XBMC you can run it like any other program, but you can also select it at login, to only start XBMC (whithout Unity).
Experienced users could ofcourse implement this themselfs (similar to the process described here: http://vexfloss.blogspot.de/2012/09/why-and-how-i-use-single-terminal.html
). But it would be great if it was alredy implemented by default.