Well yes... I mean... first off with the sword in your mouth it get's in the way of any swings you try to make with your right hand which I assume you are right handed. Then there's the matter of how do you even manage to hit anyone with the sword you keep in your mouth? Wouldn't you leave your head vulnerable with any attack you made? Also you don't get the mobility needed since you have to swing your neck everywhere in order to move it... It just seems all rather... impractical.