If you have a tank and a healer of comparable skill level in a single-target enviroment the tank is going to out-DPS the healer every time. I say this as a healer main with some DH melds myself.
It just boils down to the time spent dealing damage - virtually everything a tank does cauaes damage to an enemy where as there are almost no healing abilities that cause damage. That's not even getting into auto attacks.
Because tanks can hit significantly higher dps numbers it makes sense for them to lose the negligible survivability boost offered by tenacity in favor of perhaps making their healer cast an extra heal or two over the course of a fight because there is nothing I can put up as a WHM that holds a candle to a Fell Cleave.
The best team composition is the one that kills the enemies the fastest not the one that specifically allows healers the most time to dps. As healers it is up to us to find our dps windows by learning a fight, it isn't on our group to gimp their dps in order to let us hurl another stone.