If you are far enough ahead of your opponents late in the game, then sure, you won't need an army of panzers to take a city. But if the AI is even with you, or even (God forbid) ahead of you, they can be essential in an offensive war, especially at higher difficulty levels.
An army can also be useful for defensive purposes. But I don't tie them down in specific border cities. I let my Pikemen/Musketmen/ect. do that. But your panzer army--or any mobile army like all cavalry, all tanks--can still be very useful in a reserve role: moving up to meet enemy threats, launching retaliatory counterattacks, or making raids into enemy territory.
I don't really think a ModArm Army is so much of a waste, as long as I still have enough individual armored units to hold the line while it's repairing itself. An army of four panzers will find very few opponents that it cannot defeat.