just did a bit more testing to disprove some of the earlier comments.

Casting offensively or defensively the speed of "turning" is irrelevant. You can rotate back and forth as fast as you want and as long as you are still mostly facing the mob your cast will go off.

Same for healing accept for the whole facing the target thing, don't need to do that there.

The point of holding down right click is so you can follow a mob should it path and also keep your camera under control. An uncontrolled camera will kill you on demon wall as an example.
It is not to rotate the camera for better viewing, but to keep it centered at your back, and also to turn and follow the mob.