To me, this is not so much an interesting feature as something that got lost from MTWI, probably to accommodate "wider public" by dumbing down the game in RTW. In MTWI only the front row of crossbowmen would fire while the second row would reload. Anyone beyond the first two rows would just stand. Archers would actually shoot, but, unless on an elevation anyone beyond row 3 would cause friendly fire losses among the archer unit itself. So it always made sense to deploy them in 2 rows if on flat groun.
*shrug*. Personally I think friendly fire within units was silly back then. You don't shoot your own mate who's half a meter away from you in the back just cause "he was in the way" . Unless of course said mate cheated at cards the night before .