Eh, I have several reasons myself which is why I eventually bought one. Some good, some lazy.
I have HD Netflix in Living Room(Xbox) and my room(PS3) without having to move consoles. Bluray. ExclusivesLBP, inFamous, MGS, GOW, KZ, Resistance, Demon's Souls, etc) And some games just run better on PS3, 360 or PC specifically enough to be worth playing it on one of those. Example being Dragon Age, I watched my friend play it on 360... if it only came out on 360 I probably wouldn't have played it. Also looked just as bad on PS3, however on PC it played better, looked better, and enough of each to be worth playing it on PC. I have friends with both, one or the other, and am able to borrow games on whichever they have, so that's always nice. I mean if I don't have to buy 5 games, there's the price of it back. And really I just like having the option. I understand everyone doesn't feel the same or can't do the same, but that's my justification.