There's only a handful of games I don't mind playing over and over. Most of them are multiplayer, which usually helps a game from being stale. But no matter how much you game on your old consoles, chances are, there are still quite a few fantastic games you missed on them. I know I played a ton of great games on my PS3, but I still know I'm missing a ton. So that's definitely one reason to keep old systems. You'll most likely never play through the entire library (even if you just play the good games). That's the beauty of it.
Now that new consoles don't have BC, that's an even bigger reason to keep old systems. But I don't like to horde stuff, so there are times when you just have to let go.