I have to guess MAC gamers (if there is such a thing)know what to expect if they purchase both PC and MAC games. If they only purchase MAC games, they certainly aren't going to see a long list
Initially, yes, but if they can write once and build on both platforms, then that list will grow - providing they can convince people to use Source.
Of course, I don't think this is the magic bullet - sometimes we end up doing things on one platform one way but find that when we try to do the same thing on a different platform, it doesn't perform or behave in exactly the same way.
I think this is great though - I've got a steam catalog full of games (almost all the Source-based titles) that I haven't touched in ages that I'd like to be able to play in OS X.