With poly bushes you still got some movement and therefor play on the rear. Sure they are better than the original rubbers, but I choose to do it the right way.
You can't tell the difference in road noise of vibration between the rubber bushes and solid ones. Ford should have fitted these as standard from factory especially to the cosworth range IMO.
I'll second that. Another reason is that the alloy mounts are so easy to fit. I can imagine trying to fit single piece poly mounts without taking the beam off would be a challenge