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On a fully populated server, the best strategy for defense is to hold gates/ladders/walls/etc. to slow down the flow of attackers to the flag. On NA2 with ~10 defenders, the only viable strategy for defenders is for everyone to sit on the flag. Which is boring, so several defenders always choose to go solo on the walls and valor farm (I'm guilty of this whenever I'm on defense of a hard-to-defend castle).
It's one of the many reasons I thought Conquest was the ideal game mode; the "action" naturally gravitated toward the flags, so you didn't have to choose between helping your team or having fun. Of course the multiplier+valor system is a horrible match for conquest (should just be straight points->XP/gold, with bonuses for capturing/defending objectives at the end of each round).
I'm so sad Conquest got scrapped. It showed serious promise, combining the best elements of battle and siege without the worst drawbacks of either.