I use it for my whole infantry line. I quite often find it a very bad thing to use with cavalry because if they get hit while stationary your cavalry will get slaughtered, even lowly militias can chew-up stationary cavalry in guard mode.

Guard mode is excellent in receiving charges. Also it prevents your infantry from turning to engage a unit. If a unit turns that is in the middle of your line it now has an exposed flank towards the enemy wich the A.I. in 90 percent of the cases will pounce on.

Also guard mode keeps your troops in a fresher state since stamina is an average of the whole units fatigue. With only the front row fighting and dieing the rear ranks come in to fight with fresh stamina.