The hybrid tax is always there. They will say it's not there but it must be there no matter what. The reason for this is that if you don't have a hybrid tax, everyone will just play hybrids.
For example: druid is good in all specs "because no hybrid tax" -> "what's the point of not running a druid". Hence -> druid will NEVER be GOD in all its specs, never going to happen.
Similarly if you're going to make mages healers, you are screwed if you want to be a ranged. The amount of those rolling mages would skyrocket and eventually nerf them.