Personally, I don't find convert to be useful in XIV. Your MP ends up regenerating before your puller can bring a new group of mobs anyway. I guess it's good if you want to give your WHM a small heart attack every now and then.
In XP fights, yeah, you're right, you just don't use MP that fast. I'm looking forward to using it in Moogle, though, where I'm always having to ration my MP. Should prove useful in other boss fights as well.