[@Irredeemable] You might not care about crew mortality rates but you have to remember how said crew got killed in the first place: weapon malfunctions.

The crew is most likely going to die with something about their mortars misfiring or otherwise going kaput and if that happens, the mortar is also going to become warped and damaged to the point where it can't be used assuming it still in one piece to begin with. Your rats might be expendable, but your mortars and the price you pay to have them commissioned and fielded is not.