It's shown because the Alien is bipedal like a human.
There are quotes from Ridley (and it's the ALIEN SECRETS section).
"The nasty one, the thing that sprung out of the egg - the 'perambulatory penis' as we used to call it - is the father. All it does is plant the seed. And the next generation takes on charactertistics of whatever form it landed on."
it's exactly the other way around, actually -- the idea of the Alien adopting characteristics from the host could justifiably be regarded as a retcon. Sir Ridley had Giger's bipedal design as a starting point, and going on from that point, he thinks that "hey, what if...?"
I believe the full paragraph you were quoting goes as follows -- the second sentence giving the crucial context:
FACT: There was much more to the Alien than met the eye. Since Ridley Scott used a painting by Giger as the basis for the adult Alien, he had to work backward to create the first two phases. During this process, the director came to understand the organism completely. "The nasty one," he says, "the thing that sprung out of the egg — the 'perambulatory penis' as we used to call it — is the father. All it does is plant the seed. And the next generation takes on characteristics of whatever form it landed on." This means that the ALIEN may not always be a biped! It could conceivably be a combination of the original Face Hugger and whatever host it uses!
So, the Alien is not made bipedal in the movie because
it takes on characteristics from its host -- Sir Ridley muses that it might take on characteristics from its host because
the Alien in the movie is bipedal.
Ergo, you can't really say that "the concept made it to the film and is shown on-screen", because it actually went the other way -- the concept is a rationalisation for what is on-screen, not the basis for it.
Which brings me back to square one: going from what is seen on-screen in Alien alone, the idea of the Alien taking on characteristics from its host is one possible rationalisation, but nothing seen or said on-screen mandates it as the only
Besides, I feel the whole proposition that "humans are bipedal, the Alien is bipedal -- ergo the Alien must take on characteristics from its host" stems from somewhat faulty logic anyway. Even here on Earth today, humans aren't the only bipedal life form -- and looking back through Earth's history, you'll find they've been quite common at one point or another. After all, it's not an entirely impractical mode of locomotion.
So what's to say the Galaxy isn't teeming with bipeds -- including the Alien?