CMs have only bombard function, but they can actually kill units, unlike artilleries or BBs.
actually CM is a very effective weapon in attacking cities. I always throw 1 or 2 CMs before attacking with tanks, and I usually lost very few tanks. Considering CMs are much cheaper than tanks, it's a great bargain.
Another good use of CMs are to defend your coast and kill the enemy BBs who're near your coast. First use artilleries to reduce the BBs to one 2 point, then use a CM to finish it.
The only thing bothers me is the short range. After you get radar artillery, CMs are useless in attacking cities.