I think the point is that even if you could--and even if they were very, very tasty--they have a level of intelligence and self-awareness such that we shouldn't be doing that.
---------- Post added 2012-02-23 at 12:20 PM ----------
That's not a solid litmus test, as you're basically defining rights as being human. I mean, if some aliens just dropped by on some space craft and had no language or communication method we could understand, that wouldn't give us the green light to slaughter them as if they were worthless and had no rights.