Science Fiction has a mono-culture problem. On one hand it is a proven and reliable story telling and world building device that allows talented writers to tell deep stories which examine the flaws of humanity through actions of the alien races. This, in a show like Star Trek, allows the writers to portray humanity as the hero and the perfect society that it hopefully evolves into, while exposing the flaws of our current society through the actions of the aliens. 

The downside to the use of monocultures is that it reinforces, passively, stereotyping. In Star Wars all Radians are bounty hunters, all Jedi are selfless and all Sith are selfish. In Star Trek all Vulcans are morally upstanding and emotionless, all Klingons are warriors obsessed with honor and ritual, all Romulans are cunning, deceitful and paranoid and all Bajorans worship the Prophets.  These portrayals, albeit passively, support the reliability of stereotyping a person by race or cultural identity. 

So on one hand, the use of a monoculture allows a progressive thinking show, such as Star Trek, to tell smart, heady, deep morality plays which explore the strengths and weaknesses of current human society and the perfect society we hopefully will one day evolve into. It is both a good and bad thing, hence, science fictions problem with the story telling device of a monoculture.