I find it useful because it pairs well with Energy Drain for a quick spike on damage. My Ruin II's are about 200 with my Energy Drain at about 350. All told, I can do 600 non-crit damage over the timespan of one GCD.

That aint half bad.

Who cares about the higher MP cost? You are on a dungeon you are likely overgeared for, trying to quickly clear content, on a job that basically cannot run out mana unless you Succor bomb.