When people talk about WHM being reactive, I tend to assume that they are referring to how WHM healing spells are unnecessary except when damage is being incurred. That isn't to say that Stoneskin isn't a useful mitigation option or that WHMs shouldn't anticipate damage and act accordingly. The main difference with SCH healing is that, if you predict the fight correctly, you can plan further in advance with more preemptive shielding and even SS if really necessary.