I just love cartillery when you use it against artillery.
That's a personal fight commander vs. commander,just like looking for the white dot(enemy commander when he's looking at his laptop,in case you didn't know) on the map and hunting it down with arty.
On the other hand - dropping vehicles on single players or planes is just lame and a waste of one destroyed enemy arty gun.
If you use cartillery against the enemy commander and make it a personal war between you and him it's all legitimate,you both try to keep the ticket bleed low by taking out the other guys' artillery - with all means necessary.
Try looking for the white dot on the map and drop a car on it,you'll be surprised how pissed off the enemy commander is gonna be.