I think it's more a matter of just wanting casters in general to have an "Oh Shit" button for when the boss is about to explode the arena and kill all of the squishies. I would assume in this hypothetical that BLM would be compensated.
Granting, with Titan-Egi getting reworked from tanking to providing survivability to the caster, the only job that really has that issue is...
... Yeah, that. In which case, just give RDM a tool of its own ("En Garde", anyone?), leave Manaward where it is and we don't have to worry about SMN becoming a caster tank.
To be honest, I'd rather just give BLM a buffed version of Drain. If Manaward already provides BLM with snap survivability then it shouldn't use another cooldown to heal back up.
I'm totally fine with BLM sacrificing DPS in order to keep itself alive -- that's kinda the point of it being the archetypal squishy mage.
