as a first, the high price comes from effort that is required to obtain one. You have to complete all dungeons and quests from first three dimensions which is a lot of work.
As you have seen,, it increases ur final damage inflicted by 20% on odd turns and reduces it by 10% on even turns. So you get an average of 5% more damage each turn since u gain more than you loose. Final damage hereby does indeed mean the damage you would deal without the cloudy, multiplied by a factor.
Lets talk about pvm now. In many cases, it is nessecary to blitz a monster in one turn. A 20% extra damage helps alot here when u can use the malus turns for buffing and map manipulation. Now think of me, im a cra. I can smash all enemies with explo in first turn with increased damage for quick fights. Punitive arrow can be cast "only" every 2 turns which makes it perfect for cloudy. Iops got Sword of fate with the very same concept.
I dont wanna pretend i know much about pvp, but it should be the same factors as with pvm. Blitzing chars. doing as much damage that cannot be healed within a turn. Buffing in malus turns. Surprising with even higher damage or odd turns.
I wouldnt say its useful on any class (panda im thinking of you), but on some it certainly is.