I don't see why aliens would need powers, they're just beings from another planet, they're not supernatural.
I quite like the aliens in sims 2 and the abduction cutscene and the one when the sim-dad gives birth are very fun and cute!
Although it was fun to attack sims with spacecars, abduct them and kill them by summoning meteors, the aliens in sims 3 are still my least favourite. It was boring how they always looked exactly like the pollination technician and didn't get any feature from the sim-dad.
And like others have said, I too would like the option to remove the disguise completely in sims 4.