Marcia may have a point...
While it's obvious that multicultural societies are more adaptable and more creative, monocultures--while they tend to be more stagnant--also tend to be more stable within themselves, with fewer conflicts and social problems. Obviously, this is overgeneralized, but examples abound.
This is a heck of a choice when trying to decide who you're going to stick on a starship for a few generations.
One way around it might be to launch a closely spaced (astronomically speaking) fleet. Enough "personal space" between cultures, but the advantages of multiple ideas and worldviews when faced with a new situation.