I feel like at a lot of schools being lgbt does help a little. But only if the admissions office knows, which usually they don't, b/c they don't ask, and most people don't tell.
That said, I think there are other things that most schools find just as valuable in adding diversity.
For instance, at the school I'm going to, I get the impression that serving in the military is probably just as valuable to them as having experiences as an LGBT person. Being from Alaska might be a plus too. Or perhaps being a missionary.
The point is that I respect a school's decision to add diversity, whether it be any of the above. And while some schools do consider LGBT status to be something that adds diversity, there are a lot of other things that they consider to add diversity just as much.
On the other hand, I've never seen a school discriminate on the basis of veteran status or religion, but there are a handful of schools that maintain policies where students may be kicked out if they have same-sex relationship (Baylor, Pepperdine, Liberty, Regent, Ave Maria, ect). And there are many schools that won't allow official student organizations that work with LGBT issues, but I have yet to see a school not allow an official student organization that is faith-based.