If you use the 'g' key instead of your mouse to group units you're a little safer. I never use route (my troops seem to manage that well enough on their own when push comes to shove), but withdraw is very useful, not just for replacing missile units, but for removing any unit that's ready to retire. In a long battle heavy units get tired soon (especially if you've taken them to the desert) and you can withdraw them and replace them with fresh troops, or if there's a lull you can send out depleted units (5 or 10 men or so left) and replace them with full ones. Use it to keep your army in peak condition. The AI's favorite use of it is to protect their one-man supergeneral units by withdrawing them the moment the battle begins.

Ajax