I am suddenly worried that Siege Towers will be pretty useless in the game if you can toggle fire arrows when you want... Siege Towers are vulnerable to fire arrows (emphasized in the flash site that comes with the demo), and if every archer can just select fire arrows, those towers are going to fall pretty fast.
Or maybe this just proves the reverse: because it's unlikely that Siege Towers will be made entirely useless, not all archers have fire arrows (or maybe you have them under certain circumstances).

If anyone knows for sure which of these is true, please tell us.