Arquebusiers can be highly useful or nearly totally useless. If its raining, or if they have to face up hilly terrain, they cannot fire at all, and their value in melee is pretty limited, though they can at least serve as a semi-anvil due to higher armor and decent defense numbers.

I would like to note that this army IS defensive. The organ guns and serpentines are totally immobile, and the enemy could simply await out of range if they were the defenders. If you were attacking, one would be wise to replace them with something more useful, like Szekely.

However, I may give this setup a try sometime. Personally, I do like to use muskets when I get to the late game for flavor. However, I always take care to have reinforcements in case there is a truly terrible storm. Its those organ guns that are so dang tricky. Not just the painfully slow reload, but also the fact that after three or so shots they're empty.