The only PRACTICAL reason to /WHM is if you see yourself needing to throw out lots of -na spells for multiple people, particularly if you don't wanna be within range of eating those status ailments yourself (that pretty much rules out /DNC) and many would argue that you can get the same or better results by subbing scholar.
To add to what DB said here, /SCH is typically your sub of choice for party work. The only thing /WHM has over it is stona.

DWIII is 25% DR, suppa is 5% additional DR.
100/70 = 42.85% increase in melee DPS from /NIN
100/80 = 25% increase in melee DPS from /DNC

That doesn't take into account the additional hit on WS's nor the slightly increased TP gain speed. So for raw kill speed /NIN or /DNC are the way to go, for party support it's /SCH or rarely /WHM.