Why have aliens when you can tweak some humans a little so that they ARE the aliens!? Their differences could make them weaker or stronger than 'stock' humans or have weak/strong differences that are balanced.
That's a debate that's been going on in Traveller for 50 years almost. But I think there is something telling that two Alien Modules are Zhodani and Solomani. That is to say. Humaniti is both the Alien and the Familiar all at once. Why have Aslan when you can take an "Imperial" (Solomani/Vilani) human and turn them into a catgirl or catboy?
Mainly because Aslan are distinctive in physiology, history and culture. Aslan are more than just fuzzy humans with dewclaws. They're a People that think differently than humans. Hence, the Aslan's
Tolerance skill. Aslan culture is so bound up in their concepts of honor and face that they'll duel anyone and everyone because they
smiled the wrong way at them. In he end, playing an Alien can be fun, which is really the only reason to have them as an option in a role-playing game.
My point in the essay though, being about humans, is that all-to-often I run into players who don't want to play as human for no other reason than they are perceived as boring, or lacking any special abilities or bonuses. In campaigns where humans are the majority species in the setting, having group after group of funky aliens starts to undermine the premise of the setting itself.
D&D had this problem since it's early days as well. Why play a human when elves and half-elves are superior in every way? or Dwarves get bonuses to their Constitution? or Dragonborn get to be... well,
Freakin' DRAGONS y'all!?!
Of course, there is no right answer. Aliens, no-aliens, we are the aliens, it's all a matter of choice for each group at each table. I advocate keeping our options open, so when a player sits down and starts to whine about how "humans are booooooring" the GM can guide them to some options that helps the player find excitement in their character.
OH! and almost forgot!!! Thank you for the feedback! I really appreciate it!