To each their own. The original script sounds pretty bullocks to me. I'm very happy with the film we ended up getting.
The life cycle shown in the film may serve no "purpose", but then what purpose would any life cycle serve? To show us the alien lifecycle? Either way it exists to exist. That's not really an argument against what we got, other than you preferred more Alien tie-ins. As the film isn't set on LV426, doesn't contain the ship or specimens seen in Alien or any other film in the series, and actually modifies many of the ones that appear similar, this isn't much an Alien prequel, and I guess that is why so many Alien fanboys have so many hangups with the film. Just enjoy it for what it is.