And I do rotator cuff work, before and after my torn labrum and rotator cuff surgery. Before, I needed to in order to keep my shoulder from popping out and EVERYTHING. With the shape of my glenhumerol joint and torn labrum I could not keep it from popping out in certain situations but I can reduce the number of times. Did it after the surgery to keep the muscle from healing incorrectly.
If you are healthy with a balanced weightlifting program your rotator cuff will have the appropriate strength and endurance for your shoulders that you need. Building it up unnecessarily can lead to impingement and or stabilizing issues. The only other reason I can see to specifically work it, is for Arm Wrestling. Other than that you are wasting valuable gym time.
For the record my internal rotation strength went up drastically when my bench and Flys went up, and my external rotation went up when I did single arm pulling exercises with slight twists. I only had to do them specifically because I could not do those exercises.