you're right, camping the flag is a crap strategy, but it's simple, and doesn't rely on any teamwork to succeed.
If you have 3 people at backdoor, 3 at gate, 5 at ladders, etc etc, then when one point is getting overwhelmed the entire thing relies on someone typing that they're getting ass bitten before they die, which very few people do.
If you had an organised clan defence, with the team split evenly amongst the walls / bottlenecks with hammers, crossfires and stuff then most of the castles in the game would be literally impossible to take ;).