I haven't been to a massage parlour for a very long time, as I know that if I'm in the mood I would much rather go somewhere where I know I will get laid, than going somewhere where there's a small possibility of actually getting laid, HE aside.
Having said that, I imagine that for some guys the ambiguity whether the girl will a) be good looking and, b) if she will go all the way, is part of the appeal of these places. And there's the build up of sexual tension during the massage itself.