Let me play devil's advocate for a moment and ask the people who argue that aliens are unnecessary because they wouldn't add anything new "Well, and what do the specific human factions we have actually add to the game, exactly?"
I mean, the canon universe is nice and all...but really, Liao vs. Davion, Steiner vs. Kurita, Inner Sphere vs. Clans, everybody vs. the unimaginable might of the Tetatae Armada: it's at heart just yet another iteration of Red vs. Blue. There's nothing new under the sun here anyway.
(And yes, that's the main reason I've personally grown a bit distant from and disillusioned about the whole "canon" thing over the years.)
There are five sources of war: For Land, For Wealth, For Life, For Freedom, and For Some King's Lies. In essence,
each of the houses represents one of these:
The Confederation is For Land(their own!)
The Combine and Clans are For Life, because in both factions, battle is life.
For Wealth is pretty much the Lyrans.
For Freedom, ultimately, is the Free World's League(why else would they always be having Civil Wars?).
Federated Suns is the standard of For Some King's Lies.
On top of that, even though the Clan culture is rather alien, it, like the Successor Houses, is very human. We can
see ourselves in them. We can understand their motivations because, ultimately, they are the same as ours.
In essence, the Clans provide a situation where we can shine the light on some of the noblest aspects of humanity(the capacity for honour, for putting something greater then oneself) as well as the worst(Hubris, violence, prejudice). At the
same time, the Clans provide the ultimate culmination of Lee's "It is good that war is so terrible, else we would grow too fond of it" belief.
While one could say that the 5 Successor States are caricatures of various cultures, as we really look at them, we can
usually see that those are just the surface and the propaganda. As we look at each of them, we see they have their
own unique takes on the origins we would take them as lampooning. And, from there, it can be used to explore social
issues we have today. The Confederation, after all, was likely made into a sort of Neo-Communist state and bad guy because in the 1980's, the bad guys WERE the communists.
Finally, one big argument is: it is easier for humans to write about humans. We know ourselves, more or less. Can we
make an alien that is truly alien? Even in most sci-fi movies, there is a very human objective for the aliens. Usually wealth(minerals, water, women) or land(there are come to take over and "colonize" our world). Because we cannot
really make something with an alien mindset. So, ultimately, what the factions in the universe give us something for
conflict that we can relate to, without there having to be a pretense at trying to make it something other then
"Humans in funny suits."
