When one of my armies reinforces an army on the field my general's bodyguard is the first unit to enter the battlefield. Seeing that other units don't immediately follow, but march slowly, it has happened sometimes that my general got slaughtered as he was largely outnumbered, which in turn led to a decrease of morale in the army.
I'd believe that a general would not enter the battlefield as first unit, but rather stay behind and direct the battle. It would be great if someone found a way to put the general in the rear of the reinforcements or a way to select a order in which the units enter a battlefield.