Quoting Rose Bailey:
Watching the trailer for The Host, I’m reminded that there are a lot of movie premises that I don’t much care for in and of themselves...
...but which would make lovely Doctor Who adventures.
“You mean they steal bodies, Doctor?”
“Worse than that, Charlie. Cliovores take over the body, yes, but that’s not everything they do. They take over entire species, assume and devour their destinies. Everything good and evil these people were, everything they would have become... it’s just food to them.”
You can thank Stephenie Meyer, whose movie inspired me to ask “but why would you possess an entire species?” Because if you think about it, that involves putting your own people to work in all of the unpleasant tasks. So there’s got to be a motive besides conventional conquest.