There's no purpose for aliens. For aliens to be interesting, they have to be understandable. Their purpose has to be something that makes sense. Whether they're here to steal our women, to eat us, or to blow up Terra, without a reason they're boring.
Aliens have to be human. Oh you could have monsters like Zergs/Tyrannids, or you could have Orks/Klingons, or what not, but when it comes to it, they must have a reason.
There have been sentient aliens in Battletech numerous times. In the beginning it was the Capellans with their crazy dictator, who replaced the good king Hanse with a double. Then it was the Clans, a warlike "Klingon" race that practiced planned breeding to forge the next stock of battlebabies. Lately it's been the "Robot Race", the Word of Blake, a species willing to destroy planets to keep their secrets safe from humanity.
We havn't had Zerg, though I think the entire concept is ridiculous (the protomechs were an experiment in that direction IMO). The WoB probably were not so far from a central mind however, with C3i being brought down to Battlearmor IIRC, the next step would be an implant into their cyborg brains...
Slavering monstrous flesheatng aliens are animals, and deserve no great place in the fiction, they are concerns for individuals trapped in a cavern or lost in a forest, they are hungry wolves in the Russian winter, not nation breaking forces.
Battletechs enjoyment in the factions is because their is an understanding, a broken mirror through which each faction can see a bit of itself in it's opponent. The cultural differences, and the individual flaws in leaders are what make battletech's universe interesting. We need no monsters, they come from us. Mankind is the alien.