It's something I try very hard not to do--leave a show at intermission. In the past three years I've seen upwards of 250 shows and I can count on half a hand the number of shows that I've left at intermission. I've never done it when reviewing a show.
However, I am on a judging committee and the theaters are kind enough to comp my entry into the show. I try to honor that gift. I do so by trying not to attend on sold-out nights, but arriving at the show open and ready to receive what the artists have to offer, and being attentive and focused during the show.
So the question becomes, is it ever acceptable for reviewer or a judge to leave a show without seeing the whole thing. I tend to say yes because there comes a point at which you know things are not going to get better and staying for another act wont' help things any. Also, I think reviewers and judges have to respect their own time as well as the theater productions they see. There comes a point where if you see too much of theater done poorly, it becomes increasingly difficult to appreciate theater as a whole. It is too much of a bad thing that can make critics cynical and grumpy--two traits that no one likes for their critics to have.
I don't have a good answer for this question. I do know that I won't leave a show unless I can do it unobtrusively and in a way that is not rude. But I don't think I'm prepared to say that I'll never do it.