In a world of platform wars, some people refuse to purchase a certain console for whatever their own reasoning is. I prefer to own as many consoles as I can as sometimes games are released only on a certain system (ie: the Stories series of GTA for Sony PSP and PS2 only). But for the vast majority of games released these days, they're released across many different systems and the only difference between them is simply hardware limitations.
I own quite a few systems and once I own a game for any given system I find it hard to desire the same game for another system, unless it is far more superior than the one I already own. Even so, one example of a game I bought for 2 different systems was Mercenaries 2 for both the Xbox 360 and Playstation 2. My Xbox 360 stopped working and I wanted to continue playing Mercenaries 2, yet when I bought the version for PS2 I found that it was completely different than the Xbox 360 version.
Since most of my systems live in boxes in my closet, because I have nowhere else to keep them organized, it becomes a pain in the ass to pull one system out and pack up the one I'm currently using. Lately that has resulted in me giving more thought to buying games that I liked on the system I originally bought it for and just buying it for the system I'm currently spending time on. But I still have to get a good deal on it, no matter how bad I want to avoid dragging out the other system to play it on.
I recently acquired Bully: Scholarship Edition for the Xbox 360, which promptly stopped working due to damage and once I tried to go back to the PS2 version, I simply couldn't bridge the gap! I also watched a gameplay video of 18 Wheeler for the Nintendo Gamecube, since I already own it for the Sega Dreamcast I don't see any reason to purchase it, other than my Dreamcast doesn't always work. There are a handful of other games I've been tempted to act on my impulse, but I haven't yet.
If I happen to find a version of a game I already own for another system while I'm out game hunting (and its cheap!) I'll more than likely pick it up just to have it. But to purposely go out and buy the exact same game for 2 different systems, when there is a great chance that there is absolutely no difference between the two just doesn't interest me in the least. Sure I would like a backup of a video game on another system, in case a system goes south, but otherwise I don't see any reason to just throw money at the same game without reason.