It is, by many viewers and readers, required to be somewhat plausible, or we can't relate to it. Makeup and costumes can be more imaginative and dialogue might be a little more varied. If they could do that for Klingons, they could do it for other races. The rigid stereotyping isn't due to constraints of screen size, budget or run-time. There is no good reason an entire species has to be mercenaries or merchants or war-mongers. I'm not sure what it's due to, but I can't help suspecting prejudice, which is redeemed, the writers and/or producers believe, by the further absurdity of mixing a few dark-skinned actors into every white alien crowd. I rather like the blue ones, though.