I like to help someone kill a difficult NM and then, at the very last moment, when it's almost dead... run in and have the killing blow with Holy... That's the only use I see, really... especially since it takes all of your mana. It seems more like a 'fun' ability than anything, and that's sort of how it was in FFXI too (although it didn't drain all of your mp).

I'm not exactly complaining, though. Our other new abilities are pretty damn awesome.