They are for use against stationary targets. Mortar's in real world military usage, are not for engaging moving forces. Automated turrets, even the vanilla ones, are able to kill all raiders (with colonist help, unless turret spamming). By the time they get into range of the colonists, the colonists are murdering the raiders quicker than a mortar can fire.
Enemies move to fast to have any hope of hitting them with a mortar shot. Unless I totally misunderstood the range aspect. Their range and accuracy were very very poor. I don't know if they got changed in Alpha 15 or 16, but in Alpha 14 (I skipped 15), I very much disliked Mortars.
