A mortar firing 6 times very fast in a short space of time is the same balance as a mortar that fires constantly but slowly. You could make it so that if you fired it fast it wasn't accurate i.e. Bombardment (something you don't want landing on the same spot).
Someone spots a base and you fire, fire, fire, fire, fire, fire! You hit all over the place and you either get something or you don't. A form of Commander artillery in effect but harder to use and not as powerful or long lasting.
Chances of friendly fire would also mean it wouldn't always be the best option.
But then if say someone spots a pak you zero in and try to hit it accurately.
Also the reason people leave arty pieces rusting away is because no one gives spots
Even after you ask lots of times those other 31 people don't seem to carry binoculars...