2. Use The Time To Establish The New Batman
Any amount of time spent pointlessly introducing the audience to Robin could be better spent introducing the new Batman - that should go without saying. We definitely don't want a full-on Batman origin story again - I think we're all sick to death of those and the most recent one was only eight years ago in 2005's Batman Begins - but Ben Affleck's portrayal of the iconic character needs a devoted introductory period in the movie and having to do the same thing with the Robin character would hamper the development of the character that we are much more interested in. As an audience, we need to see what this new version of Batman is capable of on his own, in order to portray him as someone capable of standing up to Superman in the inevitable confrontation that will come later in the movie. Having him aided by a kid in spandex isn't going to give that impression. The introduction and development of Robin, if it's going to be something that ultimately happens in the new DC Cinematic Universe, should be the main focal point of a Batman movie, not a mere sub-plot in a Batman/Superman movie. If anything, he could be hinted at during the movie - that wouldn't take up much screen time - but it's too early to have him fighting alongside Batman and using up his screen time.