I think having Batman on the team is valuable because it helps keep them grounded. If it were just a bunch of godlike, paternalistic superhumans protecting us weak mortals, they could become rather detached from us and run the risk of becoming benevolent dictators. Including a non-powered human among their ranks as an equal helps remind them of what it is they stand for and fight for, and lets them include a more everyday, street-level perspective in their thinking.
Granted, GL's basically an ordinary human with a fancy gadget too, but he's also a space cop and that can lead to a lack of that kind of everyday perspective. And sure, the other characters have their own everyday lives and people they relate to, but that's largely separate from their heroing lives, and having someone in their heroic community who holds and champions that more grounded perspective is valuable.