I'd agree that a shottie doesn't appear to provide much advantage in infantry combat, with very specific exceptions. 'Spread' as you say is limited, and with spread, the stopping-power of a .12-guage discharge is going to be pretty limited beyond very close range, even if you manage to wing somebody. Given reduced stopping-power (except at ranges below ~30 feet) and vastly reduced range versus AR (esp 7.62mm variants) I wouldn't want a combat shotgun as my primary. The only real use I can see for it would be, as you say, for breaching, and possibly room clearance in CQC context, and I would see its value only as a secondary personal weapon. This is certainly not a solution to the 5.56mm debate.