While politics may enter into this, I wonder if the proposal is based on a firearms instructor or executive officer watching a YouTube video or reading a gun magazine article and believing the hype about a particular weapon. Of course, this may increase training time in that you'll have to run both a rifle course and a carbine course, but darn the torpedoes and full steam ahead if it got a nice write-up in Guns and Blammo.
Since concealment isn't an issue for patrol, the only significant advantage I see in a pistol-caliber carbine is reduced noise and flash. This can largely be mitigated by suppressors, or if that is not an option, a doctrine of donning ear pro when deploying the rifle.