They have their limitations, most notably a cartridge that doesn't have a lot of steam left past 200-300 yards, but their usefulness outweighs that shortcoming. (Most "urban" scenarios have engagement ranges of less than 200 yards, anyway.)
Their good sides are light weight, handiness, high capacity, low recoil, good sights, and cheap surplus ammo. Also, they're fun to shoot, and they have history (at least the milsurp ones.) I love mine, and I'd feel well-armed if I had to use it for social work. Ninety rounds on the gun (two 30-rounders connected, and two 15-round spares in the little butt pouch), all in a short and lightweight package that is easy to haul around and shoot.
Last month's SWAT Magazine had a very favorable article on the M1 Carbine. They were much liked by many folks who had to shoot at people for a living.