defense looses 80% of the time so there is not really a need for offence to break down the walls
Bullshit. There are maps where defenders are more likely to win and maps pro attackers. The reason why attackers win more right now is because the paper-door-bug, which makes the first round pretty easy for them in most cases.
It would take too much time to get more than 1 wall down (U probably can only reach the first destruction-level) and most castles have more walls/doors/gates to get through after the wall. Actually it wouldn't change a thing to the balance of the castles. It's just more lulz. =P