If you have the spell Aspir and you are farming I find it very easy and fun to always have a crab following me. While crabs don't cast anything they do have MP for no apparent reason. This can come in handy when farming because you can cast Aspir on it when your MP is low. This saves time and you will be able to go longer without resting.
This tip is more useful with a Dark Knight than a Black Mage because a Black Mage would have too much MP for Aspir to satisfy while a the Dark Knight can just melee the target and doesn't need to cast as much as a Black Mage. It might be OK for a Red Mage/Black Mage to try this because of their "Chain Spell" ability but I don't think it would be worth the wait time for the ability to charge back up. The Dark Knight never has to rest because they can cast Aspir on the crab than Drain on their target. You replenish your HP without resting. I think it is a good way to farm and a good way to make use of your Aspir spell.