I don't think that the idea that "casuals don't care about backwards compatibility" is actually particularly meritous.
There are huge advantages to backwards compatibility, and many of them compare neatly to the traditionally "casual" crowd. For starters, if you never owned a PS2, then b/c enables you to play a large library of inexpensive games as soon as you buy the console. I never owned a PS2, however, after buying a PS3, I have purchased 6 PS2 games, but only 1 PS3 game. Why? Because they're still great games, and they're way cheaper than PS3 fare that is, in many instances, inferior in quality.
While B/C isn't necessarily a system-seller... saying that it has no impact on the buying decisions of *informed* casual gamers seems silly. Obviously, many casual gamers don't care because they don't know. That is different from saying that they know and don't care, which is what the argument so frequently throwing around implies.