On one hand, I would agree with that.
Aside from being an interesting character (and working in an awesome city with some cool mythology), what makes Batman so great, not just in these games, but in general, is that he's got wide range of gadgets and abilities (which in video games translates into a lot of variety in gameplay), and he also has an incredible cast of villains and side characters. Second to none, in my opinion.
Spider-Man probably comes the closest in that regard though.
Though on the other hand, gameplay wise, I'm not sure there's anything anyone can do with Spidey that hasn't been done. Spider-Man's repertoire doesn't really go as deep as Batman's, you know? I mean, aside from swinging around, punching and kicking dudes, and being snarky, what does Spider-Man have to offer that we haven't already seen?
Who else is a good candidate for Marvel?
Sniktbub? Iron Man? I think Iron Man is probably their best shot at a game on the same level as Arkham.