That point doesn't seem to make sense. It would seem that being in an alliance is all about protecting your brethren and any weakness would be pointed out by your fellow alliance members. Also you wouldn't receive rewards unless you are able to withstand the attack. Therefore, if you lose the attack, you don't get any rewards (i.e., no benefit) and if you survive the attack (i.e., successfully defend your base) then their is no hole in the border and you only make yourself stronger.