Not a bad idea in itself but I think that changing the number of units on the field would shake the carefully-crafted balance too much.
I guess that an “easy way out” of the “losing a man problem” would be to change the banner from a weapon to an accessory to be carried in the slot like the sergeant’s sash… Or maybe strip it of it’s combat stats and make it occupy the shield slot, allowing a bro to use a 1-handed weapon along with it? Might be a solution as well…
Though I’d much rather see the banner buffed to make a valuable mainstay 2H weapon instead.