I suspect the developers were presented with a 'hobson's choice' over wall archers.

In theory, the battlements should not block the line of sight of missile troops on the walls, but if the battlements were dropped so that the wall archers could avoid using a 'parabolic' trajectory then I suspect that the archers of the assaulting army would also be able to target them using direct fire.

What is needed is some sort of recognition that the battlements are not an obstacle blocking vision but a shield protecting those behind it.

Incidently, I don't think crossbowmen are so badly affected by this problem. Certainly those in the rear ranks resort to 'parabolic fire' but I'm sure the ones in the front fire direct. That might be true of archers too but I just haven't noticed. I also trend to do the same as FactionHier and mass my archers in deep blocks on the walls so my results may be better anyway.