In a way, the selling point is the pool of games available. For 360 in the beginning, there was like basically Halo for backwards compatible. So, you'd have to have that mammoth black monstrosity out if you wanted to play Xbox games. While PSone and PS2slim are tiny, there's really no point in having them out / owning them once PS3 is sitting in your shelf. It may not be a "giant" selling point, but definitely one of the stronger points. (ESP early in the console life. No that both have been out for so long, it's moot at best.)
But that's just more of a side-effect of having it, I don't understand how it's a strong selling point at all. I can't see how anyone has a problem pulling out and plugging in a console to the point where they're more willing to buy a different console because of it. Now, once you've bought a PS3 for OTHER reasons I can see why the feature is nice, but I can't see how not wanting to switch a couple of cables around is a priority unless the only other version of a game you have is a giant arcade machine that's tucked away in a laundry closet. The pool of games isn't really the issue unless you don't own a PS2 (which most of the people who seem to put a lot of importance into this do) or...just...for some reason is
too lazy to pull it out. You're not lazy for using the feature if you already have the PS3, but fuck, if you ever considered the idea that "Maybe I should get the PS3 instead of the 360 because I have all these PS2 games..." there is a problem. It's a convenience after the fact, not a selling point.
I mean things like this:
I have a PS3 and haven't really regretted it at all. I can't speak against the other consoles cause I don't own them, but I will say that you'll get much more out of the PS3 if you play PS1 and PS2 games on it, especially if you don't have those systems of course.
It does account for "especially if you don't have those systems", but it says YOU WILL GET MUCH MORE OUT OF THE PS3 IF YOU PLAY PS1 AND PS2 GAMES ON IT. If you're getting THAT much more out of a system because of that (again providing you have a PS2 or PS1), then either you're incredibly lazy or the system doesn't have much to offer itself.
edit: also
What really did it for me was the backwards compatibility, since I have a sizable ps2 collection to begin with. So instead of buying a 360 and a couple games, I got a ps3 with a couple games, and then a few dozen more from the ps2 era.
what really did it for me was the backwards compatibility