Another way of saying this is that the spell is either giving Fast Healing 10+ for a minute, OR is Casting Mass Cure Light Wounds on everyone for a minute.
Sure, it's wasted if nobody attacks. The point is, it is damage ablation, and is just as useful as actual healing during a fight. Even if you want to argue it is only half as useful as actual healing... that's still a repeated 4th level spell cast at least 5 times over the course of a minute.
It is WAY OP for the level the character gets it at, compared to other similar abilities.