Arbalests are the best missile unit in the game. Use pasive arbalests when you have to defend & don't want your missile units taking many casulties(if the province is cut off from main missile producing province). Use normal arbalests for attacking & they even work well in desert if they have NO armour upgrades(never bring pasive arbalests to desert).
When I attack I normally bring 4 arbalests. I put them in front of my spear/pike. March them up to enemy in formation stop when they are in range & watch the kill ratio go green, focus fire on his better units for even better effect. If the enemy charge leave your arbalests on skirmish & they will run behind spear/pike(you might have to direct 1 or 2 units asthey tend to run off in the wrong direction). When arbalests are out of the melee area switch to manual fire & direct their fire at the enemy units that aren't in melee.
For defence I always have 4 to start with maybe a reserve of 4 if their is going to be a big battle. line them up same as for attack. When defending I prefer to switch control to manual & to put them on fire at will so they won't move when the enemy gets real close. I normally concentrate fire on the missile units first then switch to the better units. Keep spear/pike right behind them so you can fire right until the enemy is about to enter melee with the arbs(this last volley can be devasting & will route a a depleted unit,arbs & pasive arbs have good defence so dontworry about them to much especially if they are high valour). Then charge spear/pike forward, once spear/pike are engaged with enemy units pull arbalests back to behind spear(with room to fire over the spear/pike) & begin concentrating fire on enemy units not in melee(or go for AI general as the arb's will kill him quicker then any of your melee units). the slwo rate of fire is overcome by with a bit of micromanagement you can make them last for a long time.