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.