Ace, there's nothing wrong with an independent attack that does a fair bit of damage. Cost-effectiveness is there- why do Hermits use Shadow Partner and Archers use their Birds if the effects aren't worth the cost?
Because Thrust sucks and bowmen need somewhere to put their remaining points . And for Hermits, it is cost effective once maxed.
EDIT: Doom can be used on Goby Houses? 
Um....there is a point to bowmen for their birds. They have a high stunning rate and its really helpful for the long rangers.
My point is that bowmen don't have other good, interesting skills to put their points into.??Doom is useful, however, turning entire mobs of Grim Phantom Watches and Vikings into snails and killing them without taking any damage.??It'd enable a priest to train solo, which is nearly impossible otherwise.??Of course, they'd have to max Shining Ray also... and Doom would have to be the maximum duration, to ensure that the monsters were killed before they turned back.
Mike, with Mystic Door left at level 1, won't you be able to get Dragon to at least level 10 or something?
But the cost is too high 
And solo priests can always heal. What's the point of all the money saving skills at Clerics have.