Here's my current setup. I have both Mac OS X 10.8 and Windows 7 installed on a SSD, and Paragon drivers for each installed on each. (so I can read/write to the other partition from both partitions)
I have several games installed on Steam on Windows, and Steam is currently not installed on Mac OS X. Some of the games are compatible with both operating systems. I've transferred the files from one OS to another, and Steam accepts them and after downloading a small amount (less than 100mb) and they work just fine. So most of the game files will work fine on either OS. I'm okay with having two copies of some small things, but I don't want two of the entire game.
So I'd like to store the games on one OS, preferably Windows, and play the OS X compatible ones on OS X without just installing them again on OS X.
tl;dr, I want to play Source Engine games on both OSes of my dual-boot setup, while maintaining only one copy of the game files.