I didn't have much success with Mortars as a main when I was testing them, but also had little research beyond the extra range. The main problem I found was they destroyed buildings too slowly, which isn't normally a problem as long as it gets done, but it meant that while they were targeted on a building, they weren't proactively defending against defenders and taking losses.
What I have found is that they make an excellent support troop. Defenders just melt away and occasionally zapping a pesky defense behind a wall or bastion. They've quickly become one of my favorite troops, their occasional abandoning their escort mission to go haring off notwithstanding.
They also make an excellent layered offense with Siege and Ranged, exploiting the corners in the defensive tower ranges to strike without being attacked, but that takes some practice to keep them in formation.
I haven't tested them much on defense, but I like the concept of a long range anti-unit troop that doesn't risk the TC. However, I believe that such a troop would only fare well in a base running Filipinos or some other Fighter debuff. Combined with Siege and/or Gunners and a layered approach of Bastions/Caltrops/Decoys, that could be quite devastating to the invading offense.
That all being said, pound for pound, Bazookas are the best at raw destruction. If I'm going for a time hit, that's what I ask for from the Alliance. The ratio of damage to troop space is very high.