WHM already has a skill that increases their spellspeed, so by that logic they didn't even need Stoneskin 2 either because they can just pop Presence of Mind and SS everyone fast, like they had already been doing anyway. The only way for SCH to get that increase in spellspeed is to summon Selene, losing their only HoT in combat while WHM get to keep both increased spell speed and two HoTs, or getting lucky and hoping your fairy procs the trait that gets you a 6 sec increase in spellspeed. So again WHM still has the advantage, an advantage that really isn't that big of a deal anyway because both classes are probably the most balanced, as balanced as the tanks are, as in they both complement each other really well by design.

Any way you slice it, SE messed up with their implementation of Stoneskin 2 when it could have been handled much better, there's no reason to try and make up some convoluted apologizing for it.