> It's not like he's going to pull out a Bankai every time he fights someone. It _is_ his ultimate move, after all. Plus, it's also not impossible to cure, never mind treat. Also, the poison will affect his own comrades too. Kind of a double edged sword there. 

One of his comrades is 85% robot. And can live in space. 

Another _eats_ anything, even poison. Only cheese is poison. 

In your defense, Mayuri's poison never really did kill anyone, mostly just slowed them down. Mayuri himself is super easy to kill alone, but with allies he is usually strong (or with intel, but he's still weak without minions). Mayuri has more creative ways to kill than with the Big Baby's gas imo.