Okay. What I see here is a good reflection of trends in SF. Back in the 60s, 70s and 80s, Aliens were a must-have in SF. Usually, they were introduced as Strawmen for whatever issue it was that the heroes had to deal with (this is the type you most often see in TOS Star Trek, for example), or as basically colourful backdrop (Star Wars), or as expys for standard roles found in mythology or fantasy (See Babylon 5).
Modern SF, as most recently shown in shows like Firefly or BSG, as well as in literature, has moved away from Aliens in order to focus more on humans and how humans deal with whatever situation they are thrown in.
Personally, I feel that both narrative styles have their pros and cons, and both are equally appropriate at different times. Yes, a gritty, human war story featuring human combatants makes storytelling easier.
But Aliens offer you the opportunity to do weird and crazy and utterly inhuman stuff, which also has an appeal. In addition, if you posit a "unified" human race, they can provide the "other" that any opposition needs to be.
In FS, the myth arc introduced with the Ancients and Shivans adds a "larger than us" backdrop to the universe that opens up a whole lot of storytelling opportunities IMHO. In conclusion then, it all depends on the kind of story you want to tell.
Also, what Scotty said.