Balance-wise, for when you first get it, I think it's necessary to not allow multiple gals firing it in a single turn, otherwise you wouldn't ever use grenades at all, just bombard the hell out of the whole map by a crew of gals playing pass-the-mortar. Remember, Dioxine's point of balance is not to make sitting behind a smoke wall and shelling out everything you see the most attractive option - the closer you get, the more fast-firing options you have. At the point you get plasma and gas mortar shells, you don't really need to fire more than once every other turn - you flatten whatever you fired at, and it takes another turn to find anything worth firing at again.
I'm not arguing that the mortar is the optimal choice of fire support, but I find it worthwhile to use, even a little later in the game; it's a great situational-use weapon. Anyways, you should want to move on from the mortar to something like the tornado rocket mortar, or tossing high explosive packs from your uber-trained ubers.