The thing I don't think you non-healers realize is that fifteen seconds of no healing = dead players. There's really no way around that. It's just not very useful for holy anymore. I'm okay with this because I recognize now that it's really not FOR holy paladins, but trying to argue that this is really good for holy is stupid.
Any healer at all would far rather have this be something you use maybe once or twice in a boss fight, on a 5 minute cooldown, than have full mana and not be able to heal for a quarter of that fight. Also sure, there are fights that require you to move. Very few of them require you to be moving for 15 seconds, during which time no heals are necessary. I can see it now:
"Okay guys, I need to get my mana back. Going to go get some chips or something."
Fifteen seconds later:
"Hell yeah, I've got mana, and chips! Hey, how come everyone's dead?"
In case you don't believe me, go run into an instance sometime, engage a boss of the appropriate level/difficulty for your gear, and tell your healer to look at a stopwatch and not heal for fifteen seconds. See how long you last.