BLM can use well manashift, its use is not different from how you use every single spell in the BLM's rotation, you have to plan it. Like when you are in a mindless spamming of fireIVs you are thinking where to go and what to use to max out your dps you should also look for healers MP and use manashift before they need to actually ask you for mana. That beign said ofc i agree, other casters are better suited, but don't think that manashift is bad for BLMs becouse that's not true. Also if you are in a situation where your healers are asking you for mps becouse they finished all their pool, using manashift won't make any difference...