A certain American today showed me that he could chainfire four light mortars in his LABC without difficulty. A cruiser or small-ish vessel caught at short range will have little time to react should something pop out and blast him.
Mortars aren't useless. I can see them filling a role with people who are less handy at multi-tasking, or used as accurate weapons for maximum pinpoint damage on a hull. Dealing good damage before someone can bot or bat up is always a plus.
Also, psychological effects of being shot with mortars.