Seriously, I think promoting a Subaki for QR3 is more affordable than sacking Deirdre's, only because Deirdre is a rare unit.
I'll admit I usually think of these kind of things in the stance of "do i really use these enough to make it worth merging over foddering"
For example I've been sitting on a ton of spare 5*s. But I'll almost certainly merge the Morgan-F & Dorcas because I use those two units a lot, even if they have decent/great fodder.
But Deirdre? Brave Celica?
Fallen Celica? Oh no, thats straight to the fodder bin.
Obvious exception to units who just have nothing worth foddering for. See also, why I have merged Summer Elises, PA Shigure, PA Azura, normal Azura, Julia....