Shields are situational. It is strong if it performs their functions as necessary, which is increasing your eHP to survive. This is something that regen cannot do. Doesn’t matter if you can stack 999k regen per tick if you can’t survive the initial damage.
The thing is... mitigation isn’t only shields’ privileges. In a party, mitigation is an effort of a whole party & the time when noct/sch shielding are needed are usually during progs where players are inevitably going to make mistake and take damage. The nature of shields is whoever has it needs to take damage in order to consume the shield & in decent runs people already knows how to dodge those damage. That only leaves the unavoidable damage that often aren’t lethal as long as the healers are keeping the party’s health on healthy range. So when you reach this point, you may ask yourself, -objectively-: “Why do I go for the more expensive shields if players can survive without it? Is there any cheaper options?”. Yes, you guess it right: Regens. I’m not saying people shouldn’t use shields after the group’s able to play decently. It’s just that objectively shields will be outshined by regens, unless the way an encounter works is adjusted in a way to make shields necessary.
Well here’s the thing: yes, AST is a hybrid, but guess what? They can do almost everything the other healer do! Even better than WHMs & SCHs in fact. They shield better when played properly on noct stance, they heal MUCH MORE than WHMs, they lose the least from using GCD healings amongst the three healers, and their mobility allows a very forgiving gameplay to not always preposition themselves & dodging bad stuffs are much easier. ASTs are arguably tad overpowered as of now compared to the other healers. The only thing that WHMs and SCH has that AST doesn’t have is perhaps WHM’s [Cure III] spams that’s evidently very expensive & situational, and SCH’s incredibly short oGCDs.
Another niche use of shields, I’ll give you that.
This is however something that rarely players will do in real practice UNLESS the said debuff is very debilitating. I.e. e10s slapping damage downs on your DPSes.
If it’s something as insignificant as your example, healers can safely eat that debuff and proceed on to help dps to burn mobs down faster. Yes, all 3 healers can do this just fine, that’s just to show how powerful healer kits are & how insignificant most of these niche uses are.