Works in an elemental wheel, if you want the mob to become susceptible to water, you can try make them susceptible to lightning first, then casting this... their vulnerability to lightning will help this stick. Once water threnody is on, you also have a higher chance of getting fire thrnody to stick.
Recently, while fighting in Yhoator Jungle, there was a black mage in my group. The black mage had Water II, and when she used it, the enemy would be damaged by approximately 150 hit points. I used Water Threnody, then she cast Water II, but the damage was 71 hit points. Are we all sure that these Threnodies work as they are supposed to?
The fact that the BLM got resisted probably had nothing to do with your Threnody at all. It's far more likely that he/she got resisted like all BLMs do from time to time, and your Threnody just wasn't enough to help that.
From my experience, the Threnody-type spells really do seem to cut down on the number of partial/full resists from my elemental magic spells. Even when they go unresisted, the damage doesn't seem to be any greater than without Threnody on. That being said, I applaud the BRD that sticks the appropriate Threnody moments before the MB...they keep me from being embarassed by freakishly low MBs.
So, as far as it being effective...it is, it's just hard to tell when you're running back and forth trying to stick Ballad and March and slapping the mages with various Etudes. If you're concerned, as your BLM/RDM what he thinks. If he/she hasn't been paying attention, make them. ^^
Just because you can't hold a tune doesn't mean the song isn't good.... less resistance means less chance on partial or full resists. But that doesn't mean there is NO chance on a resist.