I must say that being a sentinel to only shoot the mortar is just stupid besides from doing more damage then good to the game and specially in teamplay because of the more increased chance of killing a teammate, I can't say I didn't do that and I didn't kill lots of my teammates in the process but every time I kill a teammate makes me really don't want to kill or extensively injure another in my next shot, people should understand that killing a teammate is bad and should try to keep away from it while shooting a mortar
if you want to use the mortar while being on D you can't be HD you have to be a HoF and do what the HoF usually does, being a HD does more damage than good by just shooting a mortar anywhere in your base even if you only hit your enemy in a shot doesn't mean you won't hit your teammate in the next, also you can't just be a HO without awareness to the capper because you can make the capper fail bad with the slightest push by the mortar while trying to kill the enemy HoF or by just shooting and distracting
I'm not saying you shouldn't shoot a mortar, in fact you should but with the decency of having proper timing, if it isn't the time to shoot it then don't shoot it, you have other weapons use them, well they're not as powerful as the mortar but they can usually get the job done perfectly
you want to protect the flag carrier? then do it but try to do it without shooting a mortar at their face, at least wait for them to die while seeing there isn't anyone to get the flag then shoot the damn mortar to kill the enemy who is trying to return the flag and probably shooting the flag towards you in the process
you see a chaser who can probably kill the capper and return the flag? then shoot the damn mortar at his face when you can only hit him or not killing nor slowing down the capper
you're on defense and a capper is coming to grab the flag?, then shoot the mortar at the cappers' route to kill or slow him down, you didn't hit him? no problem you SHOULD be on the flag stand blocking him from grabbing the flag, you failed?(probably because of that idiot on the tower shooting the *dancing* mortar at you) then no problem you can shoot the mortar at where the capper is going but only if the chaser is far from the enemy capper because you could just hit and kill the chaser
try to think of what is the proper situation and timing to when to shoot the mortar, if you didn't do good then you did bad