I think its to show that having decent weapons which dont bend or shatter when you use them increases the effectiveness of a unit. I imagine its something quite tricky to model, as weapon quality would have an effect - read Polybius' description of how the gaul's swords bent after a few blows and had to be straightened underfoot... Not the most useful habit in the middle of a battle...

Its certainly better than the triple upgrade you got in vanilla, where you could get 'gold' swords and shields which were just silly...