IN FANTASY ROLE PLAYING GAMES
Some purists do not like to introduce any character or monsters into their game world unless they have a medieval or "Tolkienian" flavor or base. This really limits their play possibilities as far as I am concerned, for what better world to accept aliens than ones that already have a myriad of other strange and weird creatures as residents? Sure, it would be hard for a town like Peoria or Indianapolis to accept strange alien creatures, but would it be so hard for people that probably have elves, dwarves, hobbits and the like living down the street from them? I think not, for what i stranger, the alien with the blaster or the multi-tonned dragon that breathes fire? Think about it, and I think you'll find that logic supports the use of aliens in fantasy games, and that playability supports their inclusion as well. They are fun, challenging, and very novel as characters and as monsters. I can still visualize the pair of Vegan space travellers trying to figure out how a wand of fireballs worked after they had traded their stunner for it. They ran every test imaginable, and their computer kept telling them: "This item does not compute!" Still, it worked when that funny looking guy in the purple robes sold it to them. . . .
You get the point, I think, but let me just say one final thing on the subject and we'll go on to other things: The very essence of fantasy gaming is its total lack of limitation on the scope of play, both of its content, and in its appeal to people of all ages, races, occupations or whatever. So don't limit the game by excluding aliens or any other type of character or monster. If they don't fit what you feel what the game is all about, don't just say "NO!", whittle on them a bit until they do fit.
-from Welcome to Skull Tower (The Arduin Grimoire vol II), page 99