To be honest, I think mortars should be removed.
Using the external program to calculate mortar strikes can put me and a few guys in a position to control a 1km diameter around the mortars if we have a good supply line.
No a calculator is mostly used in much more organized type of rounds ( high above any pubbing session ) - PRT and similar where any second of round matters a lot.
To be able to effectively use mortars you need a dedicated mortar squad in your team with not less than three and not more than five men. What you need more is a solid supply line cause mortars are very dependant on resources, so if your supply line is being cut off or you do not have a vehicle to keep supply line alive - you don't have mortar support. Next thing you need is a very good communication inside a team ( preferably all players or SL's on mumble ) to be very much effective with it.
Most of the arguments here are invalid, maybe with an exception of that foxhole thing. I agree on that they must be more resistant to fire but not indestructible. It should sustain constant indirect hits for some time, but this should not make anyone inside it invulnerable.
If you want to be a teamplayer make a squad that will have to go behind enemy lines, and sabotage supply routes, laze, spot or in any other way make mortar team a headache, buying to your team a time to advance if they move on the open and their position is under fire.
Removing mortars, or making them in any way different then they currently are is not for the sake of gameplay, its a just for the sake of few which had a bad experience few times.
EDIT: on a side note I think the argument of small maps ( 1 km ones ) is valid, and mortars should not be available on such maps

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