5. There Are Enough Characters Already
Batman and Superman alone could quite easily carry a movie (assuming it's executed correctly) - that should go without saying, they are simply iconic - and with the inevitable inclusion of such important supporting characters as Batman's butler Alfred Pennyworth, Police Commissioner James Gordon, the Daily Planet's Editor-in-Chief Perry White and Superman's love interest Lois Lane (as well as the equally inevitable inclusion of a major villain like Lex Luthor or the Joker), including Robin in the movie just isn't necessary and would end up being complete overkill. Having Robin appear in this movie would simply be doing so for the sake of it. It would offer nothing that the movie isn't already going to have and, to me, he'd essentially just be in the way of the characters that the movie should be focusing on. Give Batman and Superman the screen time and the action they deserve. Don't have someone else, especially not a comparatively minor character, trying to mix it up with the big boys (note that I do emphasise "comparatively" here - Robin isn't a minor character at all, as a general rule). If there's one thing Spider-Man 3 taught us, it's that including characters for the sake of it doesn't work. Learn from that, DC/Warner Bros..