I'm not talking zeppelins, keep in mind. If you actually look around when you're at your town you might see the balloon over a hill a few miles out. That's your "I'm here" sign. The defenders can spot you. Now, the attacker using the balloon still can fire until the bad guys close in on him, and he can lower it, reposition, and redeploy it elsewhere (assuming he's not killed).
Or he can just set up behind a major GV invasion and let all his tanker buddies in front of him kill the bad guys as they sweep in to the town, all the while he's shelling it. You know how you can spot a GV way out but it's still distant? Same deal here. You can spot it, but it's still only some 20-feet long.
EDIT: I think that makes the difference, personally. All the other "Jeeps with binoculars" or "troop/commando with radio" stuff is about doing stuff with no threat of being spotted. The suggestion I've presented does not shirk from the fight. There would be a definite cat-and-mouse game between you and field defenders if you wanted to use this by yourself without any backup.