you are forgetting important thing, there are death penalty certs, which most of the end game ppl have who really care about %, now if you have only 50% one and flag gives back 50%, you already lose only 0.25% from war tag in sieges zone, if you have flag up you can get EE/BP ress, so you are losing like 0.07-0.15%, thats already like no xp at all
problem right now is why non registered characters dont lose xp in siege zone -> castle owners can go help to other castles for no loss at all when they die, but when they kill people who are registered than the enemy is losing xp, which is obvious bullsh1t