3. He'd Be Out Of His Depth
In this movie, Batman and Superman are set to face off against each other (at first, at least). The only reasons Batman is capable of facing off with the Kryptonian are because, firstly, he has the brains and tactical nous to outsmart him and, secondly, he always carries Kryptonite - Superman's weakness - in his utility belt, which levels the playing field somewhat. Robin would offer nothing in a battle against the Man of Steel. He'd be completely and utterly out of his depth. Sure, if Batman's fighting against a group of twenty common Gotham City street thugs, Robin comes in handy. Against Superman... not so much. Let Batman attempt to deal with Superman on his own, without having to watch over someone who would be incredibly vulnerable to a colossal beat-down at the hands of the Man of Steel - the Boy Wonder would only hold the Caped Crusader back. So, by all means DC/Warner Brothers, bring Robin in to a future movie if you absolutely insist, once Batman is established, when the villains are at more of a street level and when it's less of a risk, but not in a movie in which he'd be going up against Superman. On that note...