What would be really cool is if you could have just one Steam folder to keep both your pc and mac games in and they would launch either way. If your are mac only, then not so cool. But if you play both Windows and Mac games, very cool because not only where there be occasions when you don't have to boot into windows, you'll also save some disk space.
If you need to have a separate Steam Mac game install from a Steam PC game install for the same title, I'll be interested to hear if there are any substantial performance differences between the game running in Windows and running on Mac. If not, for the Mac compatible games I'll keep a single install in Mac Steam and avoid the reboot.