As for the mounted archers, I think if they've got some more lightly armoured bodies there, why the hell not throw in a couple of right arms with bows too?
Certainly agree, if there is sprue space available - and those parts do not take very much of it. Also, they already did it for the 1450-1500 light cavalry set.
In any case, they are essentially an optional extra, those who don't want mounted archers can instead use the arms with pointy sticks.
Also, having extra bow arms left over from this kit will allow for mixing them with the bodies from the earlier infantry kits to provide more variety in archer units. I guess the release of this cavalry set will also result in added sales of the previous infantry sets when modellers want to mix leftover cavalry bits with infantry bodies.