I started doing rotators a little while ago. I haven't been able to tell any difference, but then again I had no problems to begin with
I think training your rotators is a prehab thing... something to save you much much more pain later.
Oh and don't forget to stretch the rotators as well, inflexibility will screw them up just as much as weakness.
Try this: lie on your side, with your bottom arm at a 90 degree angle in front of you and your head resting on it. Rotate the arm down (like arm wrestling) until you feel it stretch slightly. While continuing to attempt to rotate it, gently assist the stretch with your free hand.
This helps a lot in addition to strengthening it