Do you mean you feel the alien character gets to reveal insights about Federation diversity or that they (the alien) encapsulates diversity in their own depiction?One of the reasons I always grate against fans who want a dolphin for a captain and a Ferengi security chief and a Light Of Zetar as a ship's councelor is because I think modern Trek has generally focused too much on diversity of thought by showing it through the alien crew members and not enough by showing it through the diverse-looking cast they've put together.
Star Trek is good at having an outsider, say Data or Seven of Nine, ready to pass little words of wisdom about the human condition.

now get off my lawn.