Multiple perspective to consider ...
The Retailer: From a retailer's perspective backwards compatibility is huge. Backwards compatibility makes it far easier to sell a system (the PS3 is compatible with over 1000 PS2 games for little Johnny to play) and allows them to continue the infinite-loop money maker of selling used games (they can buy a used PS2 game for $5 and sell it for $20, only for the game to be returned 2 weeks from now to be bought for $5 and sold for $20)
The Publisher/Developer: Right now there are probably a hundred development teams that were intending on starting a PS2 project in 2008 to be completed in 2009 knowing there would be millions of compatible PS2 and PS3 owners to sell their game to. Most of these publishers/developers are now wondering how big the demand for these games will be and whether they should choose another platform (most likely Wii).
Consumers: As a rough guess 75% of gamers don't spend $400+ on a console and really like cheap games; they like platinum hits, previous generation games and used games whenever they can get them. If the PS3 is to ever become more than a niche product they need a lot of gamers they hurt today.
