the formations are poorly implemented atm.
Even the "Formations mod" that we've used in every single mod for Warband did a better job... The units would behave appropriately for their formation, use appropriate weapons, and position themselves in the appropriate line. Two of the mods that made the best use of this were Brytenwalda (which includes Viking Conquest, for they've made VC too), and one other which I cannot recall the name. Anyway, when done properly, formations can basically win or lose a battle for you, no matter the unit quality, but the correct timing for using formations, giving orders, and how to move the unit. I think that BL has a handicapped formation system due to the AI being incompetent at dealing with it, so to make a virtual difficulty they've opted to keep it broken so the AI has a chance against the player.