Ethers can only be used once every thirty minutes I believe. (I'd test it if I was in the game right now JUST to be sure... but maint. prevents that -- so correct me if I am wrong).
Regular ethers restore 50 mp, I don't know about the elemental ones. I'd appreciate a cooldown reduction for ethers for sure, or an increase in potency of them, especially when siphon MP restores roughly 100-120 MP.
Yes they help (barely) and if the fight matters a healer should probably have at least a stack on hand. Overall you're still going to generate more MP with the active/passive toggling -- which is why its a messy mechanic.