My six chanter party sings a complicated song that tends to end up in a lot of (Suppressed) buffs, one of them being this absorb shield. I always assumed that would be fine because when one is depleted, a suppressed one just steps up to take over the job.

But, today I was feeling adventurous and decided to do something new and bold like paying attention. In so doing, I saw one of my chanters taking damage (from a bear) even though she had plenty of shields stacked on her. I revised my theory: A suppressed one steps up to take over the job, but it's not used for the damage instance that depleted the previous one. In practice this means all attacks against my chanters have their damage reduced by 1-30 points.

So I figured, it should still be amazing against DoT effects, since those will only overkill a shield by a small amount. But, in doing the Devŵen bounty, a battle rife with all the DoTs from the druid spellbook, it seemed more like those don't get absorbed at all.

So, ignoring opportunity cost, how much does this really help defensively?

Not much imo - unless you want to really mitigate direct damage on allies who receive a lot of grazes (your tank(s) mostly).