of course wont spam it, hell might not even hit the 5 stacks before using it
its like big damage = healing
so you go healing-healing-healing-not much damage(use smite to build couple of stacks)-use stack and repeat
it would give mana back and increase healing for when the big damage comes
more or less a boost for healing, we still cant tell if disc gonna use the spell heal, or if that is a holy priest spell only and disc will stick with flash as usual

im kinda doubting that mana reg from archangel will be affected by stacks too, would make more sense if mana reg is constant 3% from archangel, and only spell healing increase by stack, of course archangel will have cd to make it normal instead of op...

bah all that speculation and the beta didnt start yet xD