At least you can't use him in every fight if you want do do 100%, as Yuna can't swim in that dress
Zanmato is a good way to fast get spheres for maxing out stats in the monster rena, but it would be boring to use him in the game constantly. But nobody is forcing you. You'll choose if you are a hardcore gamer, and don't use Yojimbo, or if you are a beginner and then Yojimbo could be that extra touch to help those finish a greater part of the game.
But all games have had this possibility, but it is maybe to easy to lower the chances for Zanmato in this game?
FFIX - Odin
FFVIII - Quitis - Degenerator and Selphie - The End
FFVII - Cait Sith - The End
And since I see this is a discussion:
You can raise the chances of Zanmato by never let Yojimbo get KOed, and let him do good attacks when he's summoned, (The dog attack is the only bad one) and let his overdrive bar stay high. Just give him 1000 gil every time and you should get hidden poitive score on every attack.
That hidden score will eventually raise enough to that statistically you wil probably get a Zanmato every time, but it's impossible to say (since it's hidden) how many fights you have to fight and so forth.