I read a good article on AI (irritatingly I can't find it again now to link to) which makes the point that one of the most important things the alien needs to do in order to build fear and atmosphere is to avoid doing anything blatantly stupid or weird. People may ascribe intelligence to something without it needing to really be that smart, but a failure of basic stuff like movement will immediately kill any sense of cleverness. If it bugs out, starts jittering, or whatever, it stops being a threat and becomes ridiculous. And in this key area, for me, the fiction of the tail always stays intact. As it knocks around boxes and other litter after the alien itself has stalked past you, it grounds the alien in the same environment you're in, makes it close and tangible - and therefore dangerous.