My brother has a PS2. He bought a whole load of really cheap original games which were PS1 and PS2 which gave him a great games collection that he is satisfied with. He doesn't have many bucks to chuck around.
If a PS3 came out that could utilise his existing collection there is no doubt that he would choose it over the Xbox2. If Xbox2 was backwards compatible, he might be tempted to buy the MS box and start collecting cheap older Xbox games. He has many friends in the same position. Backwards compatibility sold them PS2s.
I love my Xbox, but as a media computer and secondly as a games machine. If it was hackable as a media centre I would buy one irrespective of backwards compatibility. If it wasn't - no way, I would just buy a new PC