I use it during larger dungeon pulls after throwing up medicaII > Regen > pop PI and go into my Holy spamming. Its not a GREAT use, but it is a simple use that gives me more time to DPS during pulls. There are bosses in dungeons where I can throw up a medica II, pop PI and let regen do its work while DPS'ing, ect. You dont have to use it at 3 stacks, you can just use it as a simple free extra cure for wiggle room as well. It has a lot of uses, such as Lyrai said which is my favorite use of it. The timer on it resets so staggering aoe heals is also a great way to extend its time however it must heal some hp or it doesn't grant a stack.