People forget that the sword in Zoro’s mouth has next to no maneuverability. Try swinging your head around and imagine how hard it would be to hit someone with a sword in your mouth. There’s a reason why only Zoro using 3 swords. Everyone else is smart enough to realize a sword in your mouth is useless
Even in CqC Zoro has constantly shown using blade in his mouth to defend.
Just because it doesn't make sense from a real life stand point doesn't mean it does not exist, Zoro over the years has mastered Santoryu swordsmanship, he is a 3 sword style swordsman who has claimed He prefers having 3 swords at all times
The gap between attacks generated by 3 swords like 1080 pc, Sanzen Sekkai etc and 2 sword attacks is also immense. 2 sword style is substitution for Zoro only when he doesn't have access to three swords but some of his greatest Close quarter moves like Oni Giri, and greatest long range moves like Tatsumaki, 1080 pound ho are performed through 3 blades.