The shorter gas system in the carbine length rifles is supposedly at fault for the M4 being statistically less reliable than the 20" gun in Army service. The Army recently completed a series of upgrades and modifications to the M4 to enhance reliability, minor changes to the gas system, buffer and magazines. M4's tend to get dirtier, faster and are more vulnerable to fouling than 20" rifles.
As to the Corp's selection of the full length rifle, they should be commended. The short gun has it's place in specific scenarios, as you mentioned, but the 20" rifle exhibits superior penetration and range at enagement ranges past 100yd. A combination of short/long guns provides the optimal solution, rather than completely abandoning the capability to effectively range targets past 500m.
The scoped 20" AR rules IPSC rifle competition, where many consider it to point slightly better, with lighter recoil, than the carbine.
Should read (the 20" rifle) "exhibits superior penetration and lethality at engagement ranges past 100yd"