I think it depends on the intention you have for the movie. If you are making a fantasy film (and I include superhero movies, James Bond etc in that as well as elves and goblins), then you are setting out to construct an escapist spectacle, so it makes good sense to make the men rugged and the women beautiful. The attraction of Prometheus and the Alien movies for me is that they really explore what it would be like to be a human being sent into an alien environment, across the galaxy, and be stalked and hunted by alien beings you do not understand. There is an acute sense of bewilderment and vulnerability among the human cast. The crew from 'Alien' are just fallible workmen; emotional and mortal beings of soft skin and warm blood. The revelation that Ian Holm is a callous robot is shocking, because he represents an interloper without the fears and vulnerabilities of the rest of us. When Ripley is finally left all alone on an alien planet with an enormous, acid bleeding insectivore after her blood, I still find myself grinding my teeth and inwardly cheering her on.
This is so effective in part because the cast look like real people, who you might know. Imagine the people you see everyday, sent to do an unglamorous job aboard a spaceship. Dallas might be your boss, Lambert might be your girlfriend, Kane might be your bet friend and Ripley might be your bank manager. I find them all to be ordinary people in an extraordinary situation. Great though they are, I cannot say the same for idris, Charlize et al. They look like actors, to me.
It just creates a degree of distance from the drama, for me.
