First - I say they need sentient simply because there's a few intelligence-only fields (unless they somehow manage to figure out the maths for it too - then I would agree). Like media. I highly doubt that a machine that is not intelligent will be able to create the imperfect yet lovable music, video and other sensory experience that we can. Hell - without advanced sensors, certain things are even completely out of reach, like experimental cooking. Chess is entirely mathematics - so are most things, most likely, but the mathematics of imperfection are most likely insanely complex - and by the time a machine could understand them, I'm not certain it will not have intelligence anyway. And even if they are non-thinking, technically - they still behave, and as such they would deserve at least as much rights as the animals we live with.