If Cure IV had a longer timer, I'd agree. The global cooldown can conflict here even if you did manage to hit 50%. As is, I'd expect WHMs to be capped, if not close, with merits, Light Arts, and other gear. RDM gets 15% from traits with another 10% from LA. SCH's pretty close since they're more than likely /RDM anyway. With the GCD, you'd maybe gain a second or fractions of that at best. Could it save your tank in a tight spot as a RDM or SCH healer? Hard to say. Absolutely needed for your average fight where enmity is unlikely a concern? Hardly.