Yeah, I agree. I've never seen a good argument for 3D; best I've seen is that it dulls the colors, but that kink is already being worked out, and honestly, nothing will be perfect this early in its development. 3D still has a ways to go before it's perfected.
Usually what I see is, "It's a gimmick!" But that's not a reason why it's bad. You're basically claiming that it's useless and has no effect, in which case the absolute most extreme you can actually argue is that it does no good, but not that it actually has a negative effect.
The other one I see is when people see a movie that did it well, like Up or Avatar, and say, "I didn't even notice it!" Exactly. That's the entire frigging point. It increases depth of field and makes for a far more visually arresting experience when it's done right; that means, just like cinematography, it's best when it's not constantly drawing attention to itself. When you get so into a 3D movie you don't even actively notice things are popping out at you, it's done exactly what it came to do. You actually notice bad 3D, on the other hand, because it's either popping out too little or there are too many things popping out at you; not constantly being aware of it is exactly what makes it good.
So yeah, I have yet to see a good reason why 3D is bad. And I doubt I ever will.