Of course the drawn sword has advantage.
However, the battoujutsu in fictional Kenshin world has one good thing about it, and it's the speed of the slash after it left the sheath.
Hold back a finger with your other hand, press against it, then let it go.
The velocity of the finger is much faster than when you simply swing it, no?
That's the snap effect. You get the same effect by "running your blade against the side of the sheath".
When the sword exits the sheath, it'll "snap" at the enemy.
Practicality aside, that is.
