Depends on the FF. I remember in ff3 (the original ff3) that the spells the red mage had did so little damage that it was not worth it to have them cast. Bravely default had the same problem too late game, though I will admit I loved my red mages early on in Chapters 3 and 4 of that game. But come Chapter 5 or so, the spells they had barely tickled the enemies so I felt them useless. And back to the OP: In my opinion, don't do it. If the MC is totally worthless, the player is probably going to rage quit the game long before they are done. Now on the other hand if you did a game where everyone was horrible and the point of the game was to avoid fights, then you might be onto something.