Sorry if i misunderstand...
but if you are playing with the latest version of TW:S2, you will notice that your men wont shoot through "meat shields" of your own troops, you put between you and the enemy. Not keeping your fields of fire clear just stops your men firing unless they have a clear shot.
Sending expendable troops in to a fight first, to stop your good troops being shot at by the enemy, is a valid tactic, even if it does mean you probably end up dealing with issues of routing.