this is KINDA useful for a whm, but the drawback is ofcourse the MP hit. Even with the MP hit this ring can be slightly effective. HOWEVER the smart way to use it is to simply take it off when your reduced mp is capped so that you have a bigger mp pool AND more efficient MP at 85% or less.
Conserve MP+15 as I understand it is approximately 5% base cost mp saved.
(+15% bonus to a 1/3 approximate proc rate)
Also put a link in the Trial by Water quest comments cause it wasnt linked for some reason :O