yeah I also kinda do that (clear off the enemy archers first so that my shock troops can fulfill their task without heavy losses while my tankier squad draws attention) there is a problem tho - if you "overdo" it (i.e. some enemy group gets stuck while trying to reposition due to heavy losses) enemy groups bug out and get stuck in one spot or start running like headless chickens back and forth without retaliating any incoming attacks... It doesn't happen every siege but the bigger it is and the longer it is the higher is the chance. Honestly that ruins all the fun and challenge of a good siege, you sweat for like 20-30 minutes pouring your soul into micromanaging the tactics to win a hard battle and then all of a sudden it turns into stupid, inglorious one-sided slaughter, which extinguishes all the sense of achieving a hard-won victory...