If more games were released on it that I want and aren't better or the same on the 360 I would gladly buy more games for my PS3. It's just:
- Multiplatform games are generally (not always) better on the 360
- I prefer achievements
- I prefer the 360 controller (unless it's a D-Pad heavy game)
- I have more Live friends than PSN friends
It's as simple as that for me.
I believe the only multiplatform PS3 games I have are Batman: AA because of the free DLC, Dead Space because I wanted something with trophies, and FFXIII because it was petter on the PS3. Other than, things like Red Dead I got on the 360 because my friends had it on the 360, things like Bayonetta were simply better on the 360, or things like Bioshock 2 I just wanted achievements more than trophies.