Any healer can heal any content at the moment. With that in mind, take healing out of the equation all together. Even if WHM can heal slightly higher numbers, that doesn't change the fact that all healer's can heal everything. There is nothing that you HAVE to have one healer over another in order to meet HPS checks


So with that out of the way, what do you have left? If you accept that all healers can heal all content, than you will come to a realization. It's everything else in their kit that separates the healers from one another, not the amount they heal.