Hashishin can be very useful when defending sieges. Your outer rings of defenses quit firing at the enemy when your men abandon that ring and withdraw into an inner ring (or Keep). By hiding a Hashishin (or other small stealth unit) in the outer ring, best placed away from the gates or wall breaches, you'll keep those towers, etc., shooting away at the bad guys. That one small unit can make your defenses much, much, tougher!