Ridley had the same problem, he wanted to make the Alien do all these ballet motions and odd behaviors, none of them coming off as well as he had hope; it still, looked like a man in a suit, which is what they wanted to avoid.
That has nothing to do with design and more to do with the cumbersome nature of the suit. Winston's guys under Camereon's direction made the smart move of simplifying the suit for the movement it needed to do, and it came off fine in just about every shot it's shown in.
It's an artistic choice to do the aliens with those legs, nothing more. We've had decades of aliens with regular legs and they've worked just fine.
have those problems. In Aliens, everything was lit where they were jumping all over the place, crawling from ducts, holding onto hive resin, played in reverse, but still, all sudden, with the exception of the Queen. Why? Because how easy it is to go away knowing, it was a man in a suit. Still. You can make things fast, nimble, you can try every movement trick in the book, you still have problems when it comes to crawling, and jumping, and running, in ways that seem plausible because it is outside the limits of the human form to provide said need. You cannot make human shape jump far, especially weighed down by an exoskeleton. You also cannot have a tubed head biped with straight up and down human legs, running at you, without cuts, just straight running at you, without it looking absolutely silly. On top of all this, the human form is not the best to do what the Alien does. The Alien from the body up is terrific, but humans were not meant to run at high speeds, lunge over lengths, or cover distances fast. We are not that kind of animal, and the way our legs are structures bone-wise, reflects that.
This becomes not only an issue of keeping believability, but also coming into question just how these beings are capable of being as lethal and fast as they are with the leg shape of a man. You can make it as skinny as you want, long as you want, it still cannot overcome the boundaries and stress required of agile predators. Sure, let's assume the Aliens are built differently from the anthrocentric biped look, that still doesn't solve the problem the leg is straight up and down like a man and is not the anatomy of an effective killer. In order to make an Alien preform the work it did in Aliens, as fast as it did, with human legs, they had to cut it in quick shots, sped up, or reversed, to give the illusion of high speed. If such a creature were capable of speeds present with human legs, it would have to break the laws of physics.