I think the Mortar is a necessary weapon to be, in the hands of a heavy. You go into any server, other than public tests and play against a heavy, you'll notice that they're pathetic. You get an Outrider and a Sentinel in a duel, it should be in such a way that the Outrider is running circles around the heavy, but if the heavy did catch the Outrider, he's a dead man. And because of the mortar, that is happening. Now, people are actually thinking twice about going against a Sentinel, instead of just charging at them and telling themselves that it's gonna be a piece of cake.
The only problem I see with the Mortar is it's long range. And when that range is combined with massive damage and area of effect, it becomes easy to kill escaping Cappers. This eliminates the use of chasers(but only on small maps). What should happen is that the heavy needs to be dangerous as he is right now at his base, but not at range/when he's chasing. You go close to the heavy, and you're dead, that's how the scenario should be, and the heavy need not be any more dominating than that. Before the Mortar came, the heavy had a hard time in clearing the base, but now it's different. He can handle his own, and has no need for multiple light defenders on his team. Just for that, the mortar is useful to the heavy, to clear the base, and to be able to stand up to his foes.
I look at the Mortar as a heavy weapon, being operated by the heaviest class. Therefore, the projectiles it looses must also be 'heavy' in a sense. The large green stink balls need to nerfed in terms of range, nothing else. So when the capper uses his team to deviate the heavy and is successful in grabbing the flag, he should only be killed by chasers, and not because of a mortar launched by a heavy from his base(even when the capper is half way to his base). This way the Chasers are given back their prominence, and there will be no mortar spamming on the flag stands by enemies standing far on a hill.
As for the accidental team killing happening, that depends on player awareness. One's an idiot if he were to choose to stay on the stand and defend while having a HoF standing right next to him. As a lighter class user, I normally choose to stay in places like the front bridge, towers etc to block/MA the enemy capper and reduce the work the heavy has to do. You want to stand near the flag, and damage incoming cappers/get contagious overdrive, even though you know your team got a HoF, and the HoF mortars the stand, blame your self for your death and not the heavy.
and i know how to play 
