I tend to agree with the cheap humanoid makeup theory.
I tend to agree with the cheap humanoid makeup theory. But I salute Star Trek for at least making an effort to address the vaguely humanoid appearance of most alien races. As I recall, in the original series, one character actually makes a comment about "why so many races appear to be humanoid", because they might have a common genetic origin. I think it was the one about the Apollo dude.
Personally, I think we can have no idea what they'll look like if and when we do find them. I remember reading somewhere some wag speculating "like a cross between a tarantula and a jellyfish".