But by no longer utilizing poison against the party because of the monk, the monk has effectively made the entire party immune to poison by virtue of it no longer being present in encounters! Hah!
But seriously though, cutting out stuff you know the party will hard-counter is just going to make the party not feel as cool. A balance of both is important. Believe me, as the guy in the party who could cast Silence, I know; hard-countering every boss encounter kind of makes the boss feel lame instead of fun.