Should I insulate the ceiling of a room in an attic with an insulated roof?
To answer a question like this one has to examine the path of heat from the interior space to the exterior, which happens mostly by conduction, but also by convection (air movement) where a space isn't well sealed. In your case, I'm guessing that you have a sealed attic, otherwise the insulation in the rafters would be pointless. 

If that's the case, and no air moves from the attic to the outside world (or vice versa), insulating your knee walls and ceiling will provide some benefit. Heat transfer to the attic from the occupied space will be slowed, improving efficiency and comfort (the interior wall surface will be warmer, reducing radiant heat loss from skin, or "draftiness". 

Exactly what the financial and environmental benefit will be is difficult to estimate without significantly more detail about your climate, heating system, etc. You probably won't see a cost break-even for a number of years, but the creature comfort improvement may be apparent immediately.

Moisture shouldn't be a concern with fiberglass batts. Just don't introduce a second vapor barrier through either poly sheeting or faced batts. Just use unfaced.