Chances are that if an alien race is smart enough to traverse vast distances in space to find Earth, then they probably won't be very interested in humans or in any of the other forms of life on Earth.
What about as slaves?
Robots are far cheaper, expendable, and reliable than slaves. In a space faring economy they are preferable. They don't breathe precious air, they don't eat food, you can turn them off and store them in dark warehouses until they are needed.
A human slave take 20 years of conditioning and education to be of use alien masters. A robot can be produced in 20 minutes. It can be fed just enough information to do its job, and it has no hopes and desires of its own.
If aliens tell a human slave to dig a ditch, an alien master must stay and watch him work, otherwise the human may run away.
Robots can be preprogrammed and sent ahead of the masters to prepare a new colony(unsupervised), and the masters can be assured that the work will be done by the time they arrive.
Robots will make slaves obsolete. I'm just waiting for when Stepford wives are available in mail order packages (buy one, get one free!).