Except WHM aren't forced to use their lillies at all, at best it is DPS neutral at worst it is a DPS loss, so there really isn't an incentive to use it if you don't have to.

The problem with changing all defensives to CD to build Oath to spend on damage, people will use the defensive CDs, even if they do not need to, just to get the extra damage. This could lead to cases where you don't have a cooldown required to take a hit.

This works on DRK as, in a vacuum, the MP needed to cast The Blackest Night is the same as the MP needed to use Edge/Flood of Shadow. This means, if the shield breaks, and you get the extra cast, you are DPS neutral from an MP point of view (again, in a vacuum).