I like the way the game plays with archers, but a unit of archers usually cannot neutralize a unit of musketts, unless the musketts are unsupported and you can charge them. Once the archers are out of arrows you continue to take losses from the remaining musketts. This will force you to attack or withdraw even if you don't like the looks of either option. I'd say you need at least 2 musketts, and may want to gamble on 4 musketts unless you are sure of rain. If you do get caught with rain or snow, it may stop for a while and allow the musketts to fire. If not use them in a support roll as Widda suggests. It is important just to have them there for numbers. I think you can get the enemy to rout just by outnumbering them by about 2:1.