Add after roll vs. Add before roll

I've noticed a sharp aversion among people (myself included) to abilities where you pay a cost and add to a roll, chosen before the dice hit the table. Examples are Downward Spiral for the Mortal (1 harm for +2 to gazing in to the abyss) or The Power Flows Through Me for the Infernal (+2 to any roll for 1 string to the Dark Power). Unlike spending strings post-facto, there seems to be a conservative feeling that the extra +2 has a significant enough chance of making no difference (a weak hit to another weak hit, boosting an already strong hit, or failing to improve a miss) that the high cost is rarely worth it.

Is this just poor statistical analysis? How bad/different would it be to allow to use after rolling?

The reason that those moves ask you to ante up before rolling is (at least in part) fictionally-derived. The way Downward Spiral was envisioned, this Mortal is doing themself harm somehow: drugs, self-mutilation, etc. The whole lead-in to a Downward Spiral roll is self-destructive. To my narrative sensibilities, that narration needs to happen before rolling, to have the scene "make sense" and the self-destructive stuff not be incidental to the action. Roughly the same logic holds for "The Power Flows Through Me."

In contrast, it's narratively coherent to spend a String and say, after rolling, "While this tact might fail on someone else, you're clearly drawn to me. I have a sort of power over you, and it causes you to falter, maybe - just long enough for me to succeed."

So, those two Skin moves ask you to add your bonuses before rolling, while String expenditure lets you do so afterward.

So: I don't think it'd be bad to allow people to use those moves after rolling, so long as the narrative momentum is there... so long as it "makes sense in the fiction." Maybe you could say that people have to narrate appropriate circumstances before rolling, but can then choose to "trigger the move" after rolling?