The PS3 Compatibility page at the Ubuntu Wiki lists some good arguments for why porting Ubuntu to the PS3 would be a good idea.

This is an big opportunity for Linux in general and Ubuntu in particular to gain some userbase, and introduce Linux to a whole new group of users: Gamers. This is good to popularize Linux as a gaming platform.

It can also help make the PS3 more than just a games machine, making it suitable for Desktop applications as well.