I can see where you're coming from, but I think there are good reasons to have an ally flag.
1) It allows us to respectful of the fact that we *don't* know what it's like to be LGBT. We empathize and support our LGBT siblings, but we're not the ones who have to deal with the bullying, the bigotry, and the marginalization.
2) It acknowledges that we're a part of the community. I know this seems to contradict 1, but hear me out. Every part of the LGBT community has its own pride flag; the LGBT community is one that is united by their differences rather than their commonalities. By having a pride flag of our own, allies are recognizing and honoring the uniqueness of each segment of the community, while at the same time saying clearly "we're with you."
3) It... kinda looks cool. Okay, maybe that's not a "good" reason. Maybe a better reason is so that folks within LGBT community can identify who is safe - who they can rely on if there is a problem. It's an unfortunate fact that many within that community regularly face violence, and knowing who they can turn to can make all the difference.