It depends. In my early college days when I went to a big party there was no question - always keep your beer. I hardly ever knew the hosts anyways, and there were tons of people there, so it would have been foolish to just leave it there.
But nowadays I just do small gatherings with people that I enjoy (read: non-idiotic, fun people) and I just assume going in that what I take will be consumed. If not, so be it. I doubt there's any qualms either way, but I leave it anyways. There's no reason to be stingy with someone who invites you into their home for a good time.