I sometimes out dps other dps classes on my WHM and still keep people topped off flawlessly.
Unless you are laying down on the side and playing on a console eating a bag of potato chips, there is no reason for you to not dps in any given situation.
There are few "down time" in between trash pulls where your mana is low... then you can save your resource for heals.

Why not play ANY class to their maximum potential?