Alien created as biological pest control?

I think David said they used the Xenomorph to wipe planets clean.  It eradicates all fauna, the meat, within months, leaving plants alone.

The Xenomorph makes sense as a Perfect Weapon if it has a limited Life-Span.

The Black Goo appears to be a more limited one, (more destructive) unless again it has a limited Life-Span

really depends on how long the eggs can remain dormant for. I know the egg and hugger have a symbiotic relationship so is there any canon details on how long the egg can sustain the hugger when there are no hosts around? alien suggested that with the right conditions such as the blue mist preserving the eggs they can last for thousands of years yet the route we seem to be going down is going to wreck all that. it all really depends again on where RS is going. its a very affective weapon but if the spores/ eggs can remain dormant indefinitely until disturbed, it really leaves the planet uninhabitable unless the engineers have something to reverse the damage