But it's pointless to use crossbows that way when you can use bows. If you are going to be forming lines, you may as well be able to fire a lot of arrows rapidly, as opposed to a few a minute. When you are firing in volleys, the point is to fire in unison and pretty much rain death on people. You do that more effectively if you are firing rapidly. And if you are going to be forming lines, then you are presumably firing from afar, which would be negating the armor-piercing ability of a crossbow, and as a result be negating the one trait that crossbows have over bows. So you'd just be handicapping yourself at that point. You'd be better off with a bow. You can even slap on some armor-piercing heads to the arrows.

EDIT: The one redeeming factor is that crossbows are easier to master, so you don't have to spend as much time training people.