When you have a toolbox with so many toys in it, it's really not that unusual to find one or two that's broken. But considering how Healing Rain is still a broken mechanic for shamans, I wouldn't expect any HW:S love for a very long time and actually consider it a blessing.
Unless you'd like to wake up one day with HW:S doing twice the healing and PoH healing for 5K.
Druids are already very strong healers on live, and yet they're still making incremental improvements to the class. I don't see why other healers couldn't get the same treatment. If they can get HW:S to be on par
with the other Priest heals, Priest healing wouldn't actually go up under most conditions. It would just be redistributed. You can only cast 1 heal at a time.
HW:S and Healing Rain both need to be changed. They're very expensive, short AE range, non-tracking HoTs. Every spell is going to have limitations, but I think in this case it's too much. 10k is a lot of mana to spend on a spell that often doesn't even work, and doesn't function as a "life saving" heal. HoTs are supposed to be efficient to compensate for that fact.
I agree it's virtually useless but I doubt Blizz will do anything about it, it's not as if Holy needs a gap plugged, they're still super solid at the moment.
Yes Holy Priests are currently "good enough", but Blizzard is all about incremental improvements. Out of the things they could improve for Holy Priests, I think HW:S should be one of them (along with tank healing). It's a brand new spell, and it has an interesting mechanic, but it currently isn't working. It wouldn't be a hard fix. And as I pointed out above I don't think it would even be much of a buff. Most (good) Priests aren't using HW:S at all right now.