I have never left a show at intermission for exactly the reason ghostlight stated: I always have hope that it could get better/more interesting in Act 2, which sometimes it indeed does. And also I agree that you can find almost always find redeeming elements, even if it's just one single thing, to enjoy about a show you have either paid to see or have been comped into but have already spent time seeing part of. That said, I don't think it's inherently wrong to leave at intermission, but I do take issue with someone leaving at intermission and passing judgment on a whole show. If you saw only Act 1 of a show, you haven't given it a fair shake and don't have a right to judge the whole package, as it were. You can comment on Act 1, but if you say the show as a whole is bad, you haven't given the cast and creative team enough of a chance to prove themselves, I don't think. Sometimes, an odd choice might have a surprising payoff in the second act that you don't see coming, or the book and score might just be stronger in the second act, etc.
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim