When they released the spell, they kept talking about how it would be Ret's self-heal, and how it would be a great defensive tool for Prot. So this being relegated to a Holy Paladin ability is kind of against the reason they created it in the first place.
Blood DKs are supposed to be OP with self-healing. That's the entirety of their tanking mechanic.
As it stands, this is effectively a Prot survivability nerf. It has some rather heavy repercussions, though. Guarded by the Light will most likely need to be changed. An overheal shield won't see the light of day with a 20 second CD on WoG. Holy has no issues with this, as one of their mastery buffs negates the CD. This is a massive blow to Ret survivability, which was low in the first place.