If a PD either doesn't have the money for a carbine, or doesn't allow them to be taken out in a squad car, then I don't see why they're going to buy this or let this be taken out.
If a carbine hasn't been cleaned since qual day, then chances are their pistol hasn't been cleaned since qual day either. If the optic on their carbine doesn't work, then I don't see how an optic on this is going to fair any better.
Anyone can think up strawman arguments for this, but it still doesn't serve any practical purpose or usefulness that a shotgun, SMG or carbine already fill. It doesn't even fill a niche role like certain SMG's.