Even if most of the team stays at flag, there are always a few who sally out and fight outside the walls, some who brave their way to close the gates and delay the attackers those extra seconds that will enable victory, but i dont think that these if a guy manages to close the gate half a second before the flag is brought completely down, that should make the attacking team not be able to capture the flag, the castle is lost already, it doesnt matter if the gate is open or closed, you've already lost the castle...
Or sometimes a defender tries to sally out and fails letting an attacker get in and get to the flag without breaking the BD, why should this guy not be able to capture the castle? Sneaking in is a perfectly valid tactic (and much more efficient and useful than a bloody frontal assault). Of course you also have the griefers who open the gates for the enemy, but these usually get spotted and reported (not punished though due to the lack of siege admins, koldborns campaign goes on), but lately it is usually more the attackers opening them from inside than griefers.
And most of the maps would have the same areas used even with this method, since as i have said a few times already, it would only force people to use their heads (which if they do now they usually win or at least put a good fight), which is just dumbing the game, not a good thing imo, plus it would punish those who try something that looks impossible but that for some reason every once and then works (i remember once some time ago i just went over a wall with no defenders, got to the flag, stayed there on my own for a while, then my team won, and noone except me knew what the hell just had happened, and i can tell you, the feeling is great, that of saying you people just got pwnd)