This is like saying DRK and WAR don't need provoke because it's an optional cross class or SCH and AST don't need to use protect. Some actions exist on the cross class to prevent them having to make the same skill for each individual class. Also by your logic WHM should dps as they were given Cleric Stance through CNJ.
If you was solo I'm sure you would use Cleric stance and why do you you do this? To make the fight go faster and save having to cure yourself more. Same thing in party's you DPS where possible to aim to push past mechanics making the fight go faster and smoother.
End of the day though you are right you don't have to dps as healer, but don't complain when another healer who can use every skill available to them gets chosen for content instead.