Motherhips are heavily armed ships which have high durability and firepower. It costs a lot to build them, so it is very difficult to use them in every fleets.
Even this monstrous ships have a weakness: low speed. literally, they can't avoid from anything, especially, they are easy targets of cannons due to their -40 cannon avoidance penalty. So, it is good to add something to reduce the penalty or place them not to against the cannon ships.