Though, at the same time, the suggestion increases mitigation for scripted mechanics, via putting more emphasis on Tanks having skills to mitigate the damage, meaning it can be less likely that those instances cause wipes (I.e. Decreased difficulty due to increased ability to affect them)
To say nothing about the very notion of "Giving DPS a chance to die due to random damage" - I've already mentioned in my OP about not making random damage too frequent. It shouldn't be possible to die due to random damage unless you're specifically trying to edge as a healer by letting just a single Medica II cast fully heal everyone over its duration after taking a big raid buster that put people down to 10% life (Which only really occurs in Savage+). Or in the case that this random damage has occurred, edge that target with simply a quick Regen cast when said mechanic that targets them is coming up.
In this scenario, you don't need to keep everyone topped up. Only above 50% health, since that's as hard as this random damage will hit. (Healer basics 101. People only need as much health as to survive the biggest hit they will take. Any more is overhealing)
The instances where Ramuh does AoE that hits everyone, give plenty of time for you to spend a GCD or 2 healing that target from 50% up to a level where they can survive the Judgment Volts (Which can be decreased by having said person also pick up another stack of Surge Protection), he doesn't insta-cast Judgement Volts out of nowhere, he sits around going "IMMA GONNA CAST MUH SPELL" for a fair while.
Even with random damage sources, you still plan around the scripted stuff, since those are still going to be the lethal damage sources. You just also factor in random damage to maybe consider tossing a heal over to the person if you know one of the scripted events is going to occur soon.
Like, the above scenario where you mention, a player is hit down to 50% life from a random attack and lets say this for some reason happens right before a Judgment Volts cast and the Healers are both braindead and don't cast a GCD heal to bring that player up... Well, that's the scenario that oGCD's would be used. A buffer, panic button that enables you to still get some healing in, in a short period of time.
If the concern is something like random damage occuring say, during a Tankbuster/Raidbuster which causes an insta-gib of a player even when CD's are used, then it's simply a case of making it so that random damage events can only occur during set times. I.e. X seconds after a "Buster" mechanic up until X seconds before a "Buster" mechanic.
If the further concern is in normal mode having random damage plus a failed mechanic which doesn't outright one-shot, causes a one-shot, it's simple enough to tune the damage of the random damage in normal mode to not be too high, just like certain mechanics can be failed in normal mode and not be a concern due to their damage being tuned really low.