Also means a lot of lazy ports and devs taking less chances in exploring a consoles hardware potential. The Playstation 3 had an attrosious start and devs were intimidated by its unfamiliar hardware. It was only after people started to get their heads around it that more and better products came.
If, however, Bungie had been required to put Halo 3 on PS3, Naughty Dog had had to put Uncharted on 360, and so forth and so forth... we'll, look at groups like EA
stuffing their games onto every platform they can, in as short a time as they manage, usually at the cost of quality... do you want all devs to work like EA? xD
Besides, nothing wrong with exclusives... I wouldn't waste my money on a PS3 if I could get Resistance 2, inFAMOUS, and any future games that catch my eye if I could get them on 360 :) and with some of those games most likely made primarily on the PS3 hardware, a mandatory 360 port would probably be... sloppy.