The Alien is supposed to climb into and out of ducts and ladders right? If we went with the humanoid legs, would it look like a guy climbing into a duct? And when it was running at you, would it look like a beast out of hell or an Olympic runner? That was mentioned in the interview, that they did try it out with humanoid legs first, then changed the design cause it wouldn't work to bring the terror. Having them legs would affect how the body is balanced when moving or sprinting. Frankly I would need to experience the real thing for myself before I'm totally convinced, but I expect that it would be kinda like having a small T-Rex charge at you.