USA Captain
I suppose if you speed up the thrust that would work as well. My thinking was that it should be easy to stab with a weapon you are already holding if you were suddenly attacked. This of course was possible in 1862 although it would have been rare but so were bayonet charges. It would help if you could quickly switch to melee mode from any other position such as firing or reloading. Perhaps a function that had a weak melee attack from another mode which then puts you in full melee mode until you turn it off.
I think in part what encourages the bayonet charges is the ability to rush up and stab a player by catching him while he is not in melee mode. Then again I'm new to the game and still getting used to the controls and mechanics so maybe it's easier to switch that I think.