I think the reason they keep putting Batman in sets is because of the potential for vehicles. The Flash, Green Lantern, Superman, Wonder Woman, and most of the DC super heroes I can think of get around on their own. There's not really any potential to build anything to go with them. Sure, Green Lantern could create a lot of stuff with his ring, but we won't get a set like that for the same reason we wont get an invisible jet set (no matter how pants-creamingly great that would be). Green Arrow could have a jet (I think I remember him having one), but that's pretty much it. It would be cool to see Lex Luthor get some more cool stuff, but with the Man of Steel sets out, I don't think they'd throw in any more Superman sets for the time being.
There's always the option of using a building as the focus of the set with the heroes and villains fighting around it (http://www.bricklink.com/catalogItem.asp?S=76005-1
), but you have to remember that their primary market is kids. Kids suck up the stuff that goes "vroom vroom," and Batman just has more of it.
You can tell they're at least trying to give us variety. The old Batman line consisted of only Batman characters, so it's good that we're getting some different characters.