I'm curious - are you referring to medieval armies in general, or the English army described here? What I've read of the Hundred Years War, at least (the only thing in that whole period of history which I've taken any real interest in, honestly - I could tell you all sorts about the Roman Republic, though

), would seem to indicate that the armies were somewhat disciplined and well-ordered, that every Englishman in France would have a horse - archers, men-at-arms and all - and that that name given to them does indeed indicate their role; English archers were, in fact, archers, and men-at-arms tended to function as heavy infantry. Could be that I'm just misunderstanding stuff, or that I need more reliable books.