Unfortunately for your argument I just recently got my complex damage mods, and yes they make a difference. I'm not talking theoretical, they make an actual difference. As i've said there is a reason why nearly everyone stacks damage mods and not shield extenders. Add to this that a quick duck around a corner lets your shields come right back means that for most it is preferable to go with damage over shields. Your number of 10% represents a single complex damage mod (pre 1.8). Many many many people run 3 (most likely 2 complex 1 enhanced).

Granted you do speak of scouts, and I don't know anything about scouts. But as an assault, if im stacking shields instead of damage, i may as well just drop all my guns, grab a rep tool and logi suit, and avoid combat altogether.

CCP is also nerfing damage mods in an attempt to increase ttk, which speaks at least some to their effectivness.

Just off the top of my head, away from my system, if you have 3 damage mods, with 2 of them being complex you are looking at around 16 point something % damage bonus (100%, 50%, 25%? Is that how the stacking penalty goes?). My GEK is doing 30 something hitpoints of damage per bullet without any mods, so 2 and a half rounds cancels out your complex extender, and I can put out 2 to 3 rounds in an insanely small amount of time. That is why I would imagine most perfer damage to shields.

Disclaimer- This was my first serious attempt at Dust math, and it was off the top of my head. Lots of ballparking and rounding, if im just completely off please let me know.