For a while when I first started playing MTW I kept my archers only in defensive armies, but gradually as I was forced to use said armies in desperate measures I switched to keeping archers in all my armies. I really love those hybrid units the AI has such trouble using, like Bulgarian Brigands and the like. I keep 4-6 units in my armies now, and while they don't often claim alot of kills they do provide fear as well as "free hits" against an enemy army. Archers make any defensive action easier and are useful for denying an enemy defender the advantage of a position. Plus in the case of hybrid units, they can be great to roll around the flanks of an enemy to attack once their ammunition is depleted.