The Mortar Pit is a useful defense because of its range. Although it only does one damage, when it has several defensive structures in front of it, the Mortar Pit can quickly help to destroy any enemy units, forestalling massive enemy invasions into your territory.
Never leave a Mortar Pit at the edge of your map as any incoming powerful enemy units can easily destroy it without any difficulty. Put the Mortar Pit behind Barbed Wire or a combination of Barbed Wire, Bunkers , or Watchtowers to create a strong, interwoven defense. Even as enemy Battle Tanks rip through your Bunkers they'll be weakened every step of the way and have an exceptionally low health by the time they fight past your Mortar Pit allowing your speedy APCs to finish them off.
Have no fear if your Mortar Pit is destroyed. Although it can be costly in terms of supplies to repair, the fact that you have severely weakend an enemy invasion makes up for the price.