Summoners probably won't benefit much from it since their threat builds over time. Lucid Dream will be much more effective since it halves total threat generated. Red Mage could be the same deal since they seem to have a lengthy build-up to burst. There is potential for Black Mage to need it still, though it is possible their opener starts in ice to get those umbral heart stacks first. All in all I think casters may be fine to forego it if they are taking Lucid Dream.