[Carl]

I wonder if you can fight with other UFOs. assuming that the game pulled content from someone who has a ufo, you could concievably meet one with you own, and then fight it.

also, someone else's ufo could destroy your home world

[StratComm]

I think the "sandbox" approach that the late game takes would be severely negated if you had to worry about other NPC UFOs blasting your homeworld to bits.  I don't think competetion is supposed to scale past the city level, to be honest.