I don't think she's what holds the League together, I think she can hold a League together. It's a part either of them can play, which is why both of them had a League in the Era of A Thousand Justice Leagues (which was entirely hilarious, because they got along so well and totally met each other as equals whenever there was a cross-League bash, while Atom usually flipped out and jostled for Alpha). It's also a role that isn't really needed much with the Big Seven, which I think causes it to be overlooked a lot as a primary strength in both their cases.
Man, now you've gone and made me miss Ostrander on Manhunter.