WHM here, the 20% free proc is just there to offset WHMs' lack of MP management. It's an afterthought. I do feel that leeches should have an identity that sets it apart, making it instant cast as an option. I think the spell should be reworked and taken a bit more literally.

You're throwing leeches on someone, aren't you? Have the leeches stay as a buff on the target that trigger when they're afflicted in the form a heal depending on the duration the leeches have been on the target. For every 3 seconds the leeches are active, a stack is placed on the target, up to 10 stacks after 30 seconds for a 300 potency heal.

Just SOMETHING that makes it different from Esuna, preferably something small to make up for it's group healing weakness.