Honestly, as a WHM main, I see this mostly happening with SCH co-healers during trials. There have been plenty of times where I've ended up running low or out of MP trying to recover from huge AoE blasts in Susano normal, meanwhile the SCH is just DPSing away like nothing's wrong. The fairy, of course, just isn't keeping up at that point. I would think they would at least try to cast something. Personally, I tend to DPS as long as I know I have the MP and time to do it. If sudden, unexpected things happen, I don't hesitate to immediately interrupt whatever DPS spell I'm casting and start healing. Another thing I see is a lot of co-healers that just will not hard-cast a Raise no matter what the circumstances are. As long as the situation permits I hard-cast it if Swiftcast isn't up.