Why did you purchase three copies for yourself? I have to give my honest opinion as well, which is that you sound like a paid shill. Sorry to go off topic!
I bought a copy for each system, 360, PS3, and PC. I have no association to any of the developers, publishers, reviewers, sites etc... (well I take that back, I used to do news reporting for planetdoom.com, but that was like 6 years ago, lol)
Why? because I've got some friends on each system that aren't fortunate enough to have every console or a high end PC to play a game on, and I would enjoy doing some co-op or MP with each group of friends that can only play on one or the other.
I also like trying out each version... see difference/additions/bonus stuff/dlc/etc. Also in case one system gives me problems, I have another to play it on
Don't get me wrong... I'm indeed the lunatic fringe of gamer... but I like options