2. ...And He's GREAT for Group Dynamics
If Joss Whedon proved anything with The Avengers, it's that - while watching people talk about what just happened/their next course of action isn't inherently interesting - watching them
argue about it can be damn near as entertaining as seeing them slug it out. It's no accident that Nick Fury makes an appearance in nearly every Marvel film leading up to The Avengers. He's a great character for the other heroes to interact with, and provides a consistent baseline of how one of them can be expected to react against someone who challenges their world view. Batman does this as well - even though the Justice League is full of heroes with similar ideologies, they have wildly different attitudes and methods that frequently cause catastrophic rifts in the League. Batman showing up in a few JL lead-in films could lay the groundwork and gives us a good taste for this sort of group tension. Remember how conflict is the source of drama? Of course you do, and this example is actually something that everyone already understands even if they don't think they know it yet. It's the reason that
12 Angry Men is a timeless classic and the reason why
The West Wing was a riveting television show even though both were about 90% exposition. Both understood something that is a crucial underpinning to any ensemble endeavor, and the main reason that "in the know" geeks LOST THEIR MINDS when Joss Whedon was announced as the director for The Avengers. Group dynamics are the ballgame in a movie like The Justice League, and Batman is a fantastic personality that can play off of any other member in fascinating ways. And DC/WB should milk this for all its worth. But the #1 reason why this should happen is less about the League and more about Batman himself...