Yeah, it's clear they're intelligent, but there's always the "insane ones" constantly hinted at in the novels and comics.
Besides, if they can't use their mouths besides communication, how will they eat(nothing to do with mandibles)? But that's probably a different subject.
As for the headbiting, that's true too. It'd be nice for an Alien to rip off someone's throat with it's inner jaws and watch them bleed to death, though.