As it stands healing magic skill has almost NO use outside of the standerd chance of being interrupted and very MINOR increase to cure potency, but let's be honest, unless it is a significant amount added you won't notice it, and obviously no one would ever THINK of meriting healing magic skill, my suggestion is maybe give this skill some updates to make it worth using, or rather, notice that it is actually there, because someone with say 105 healing magic skill isn't going to see a huge difference from someone with caped healing skill (which is actually hard to cap)
At 1st i thought this affected the chance of doom being removed, but i have seen no substantial evidence that it helped it in anyway.
my suggestion? raise the amount of cure potency healing magic skill has on cure spells. since it is apperent that you will not raise that 50% cure potency cap at LEAST help WHM raise it in other ways, MND helps but since it is already added along with potency in most gear it isn't that big a deal.
If not healing then as i said above, maybe the chance at removing the "chance to fail" spells like cursna for doom and later on amenisa that i read about before. SOMETHING to not make this skill a waste of time to skill up (tho it will naturally anyway with enough curing]
or maybe even effect the cure stoneskin effect from solace raise based on healing magic skill.
If you can fix summoning magic skill for SMN that used to be useless i'm sure something can be done about healing magic skill.