The backwards compatibility is one of the selling points for me for the Wii and 360 because I was never interested in buying those consoles, but with the 360, there are some games coming I want to play, plus I can play the 2 or 3 Xbox games I missed out on by not having the system. With the Wii theres the same thing, plus the ability to play old school SNES games that just aren't as good with an emulator.
With the PS3, as I already have a PS2, its more for convenience. Also, my PS2 has lasted longer than anyone elses judging by the whining about how badly built the system is, so it should give out at any time. (I got it in 2001)