honestly, B and C only make sense in an ED world where weight and nimbleness isn't a defining factor of combat effectiveness. Ergo, ED needs competent turret options for big ships before they can put on weight without reducing combat effectiveness.
I think you are forgetting those that cannot afford to A-rate a ship, and those occasions where optimal modules are not available in outfitting. Just because the Cobra is available doesn't mean the Sidewinder has no use