Surely at times it had some affect?
We don't see anyone with bayonets on though.
I was thinking it'd be: fighting in open terrain, choose rifle without bayonet for more accuracy.
fighting in close/urban terrain, chose rifle with bayonet ready.
If what you say is true, accuracy could be changed anyway so that you don't get everyone armed with bayonets.
It's always a little silly when you want to stab someone and you have to put the bayonet on.
Plus if you were fighting in a city, it's likely you might know you were going to need your bayonet. Instead of ohhhhhh shhit one-on-one duel, used up my ammo, better fix my bayonet while running around in circles!