Emergency Tactics should include instant cast

[Remilia_Nightfall]

In raid content, with a WHM or AST with you, I find it rather useless.

I have stated this when the skill was first announced and I am going to state this again - shields are almost always better then heals, for the simple reason that they can safely "overheal" without waste.

You mention "big aoe damage", but when that happens, the WHM or Diurnal AST is already casting his AoE healbomb(s) with Regen attached. There is zero reason for the SCH to healbomb as well and risk overhealing, or let the partner's Regen become overhealing by topping off people too soon. It is much more MP efficient to use standard Succor and put some shields up. Same goes with Adlo - there is no reason to use the equivalent of Cure II when you can use Adlo. Only one instance comes to mind - DRK Living Dead. But it is extremely situational.

If I need to top up the tank for whatever reason, I drop one Lustrate charge and then normal Adlo. Much better than trying to find a use for ET, imho.

ET had to increase healing potency by 50 or so. That would make the choice worthwhile: do I need mitigation, or do I need a heal that is slightly more powerful than mitigation?

If heal and mitigation have the same value, no sane person would choose heals.