Exactly what I'm getting at. Where I suggest WHM having a Lily Spender that applies a barrier, it has to be considered that the barrier is weaker than the SCH/SGE equivalent, and also has no heal attached. It'd be much more efficient to use the Lily on healing after the damage, but the option is there in case it's needed (eg the SCH is dead and you need mit to live through a raidwide). I also feel that, with Lilies allowing for damage refund, that it's good to have this 'emergent gameplay' for groups to allow the WHM to handle applying certain shields, to save healer GCDs sometimes. But regardless of that, the barrier would have an opportunity cost: It is gated behind a resource with a 20s recharge (so you have to pick and choose where and when to use barriers), and it is not as strong as the Pure Heal alternative.
To that end, it reminds me of what I wrote a while back about bringing back Nocturnal Sect, wherein AST would have only one way to apply 'barriers', and that's via their standard GCD (which should probably not be called Aspected Helios, if we're in a nighttime-related stance). So in a WHM/AST comp, the WHM would have to rely almost entirely on the AST to deal with % mitigation effects, and the AST would want to rely almost entirely on the WHM for applying Barriers, and more of the Pure Healing (as the OGCD healing throughput for AST would be a bit lower in Nocturnal).
See, these are especially interesting designwise, because the WHM and AST ones don't actually reduce the damage taken. If the WHM extends the party's Max HP by 10%, that extra 10% buffer has to come from a heal somewhere. If the AST staggers 20% of the damage taken to be dealt over the next 12s (5% per DOT tick), then that DOT still has to be healed through. Barrier healers, meanwhile, actively reduce the damage taken. That Sacred Soil or Kerachole reduces damage taken by 10%. That 10% is deleted from the game entirely. Also, AST staggering damage out would synergise with the, what is it, 3 different HOTs in their kit?