This was a suggestion by the design team to those who felt that it was too powerful out of combat.
Someone else had suggested (I don't know where, and can't give credit) to only allow it to be used 1/round, but the caster chooses if it triggers when someone goes through (preventing enemies from using it in combat). Mathematically it comes out slightly worse than Prayer of Healing outside of combat (assuming level 2 spell slot), but since you can choose where the healing goes, rather than spreading it out, it's still worthwhile. Obviously in combat it's almost as useful as it is now.