I agree with you that "realism" is overrated, even in the very very few cases in which it is actually present (as opposed to the patronizing faux-realism favored by the Grim & Gritty crowd, which is my main problem with the grayish-brown trend in shooters), however, an Avengers movie that crosses over three film franchises with different rules gives me pause.
See, I can suspend my disbelief no problem. I'm a huge fan of Doctor Who, anime, Nintendo games, Star Trek, all that stuff. However: I'm worried about INTERNAL consistency. I don't mind "The Rules" being different, but I don't like seeing them changed whenever it's convenient. As much as I loved Buffy the Vampire Slayer (and believe me, I did, and I still do), there were times when I wanted to scream, "WILL YOU KEEP TRACK OF YOUR FUCKING RULES ALREADY?"
And that's what I'm worried about. When you mash up three franchises with different Rules, you run the risk of undermining the rules that each individual one follows, and you wind up with a clusterfuck, where each individual movie in the three separate franchises becomes ridiculous because one of the other franchise heroes' powers could save the day instantly. The audience asks, "Why doesn't Iron Man just call Thor for help right now?" when Iron Man is in some situation in his own movie that would only be able to create tension when his own movie's Rules were being followed, and so on.