WHM are very good at DPS. In my opinion if you are not nuking a little at least during primals, dungeons, etc. then you are wasting peoples time. IE the ton of damage I can put out in CM would have to be made up by a DPS while I am just sitting there with full MP casting Regen on a tank. Or being able to take Demon Wall down before bees pop. Throw Regen on your tank and cast a few Holy. Stone II single targets. Cleric stance back and forth. Holy the plumes in Garuda. DPS Titan all the way to phase 3 and help on the rocks. Ifrit, lets not go there he can be beat with 8 WHM. Gear will effect how much you can DPS due to Cure and Regen strength. WHM A can DPS a little longer because his Regen will proc 150 more HP then WHM B kinda deal.

Go to CM and start DPSing. Not saying WHMs are the reason for fast runs but hundreds of thousands of damage done by WHMs instead of the same damage having to be done by DPS extending the fights does add up.