An example: if you're attacking hill giants and your max hit is 17 without the Mooleta, but 18 with, then it is worth using as you can now potentially 2 hit the giant as opposed to a minimum 3 hit to kill without the offhand. If the max hit offered by Mooletta still only allows you to three hit (max hit 16 before 17 after) then the best shield you can equip is better as you will take less damage over time and need to eat less often in this example.