Lizard said:
But WHY not?
Never mind "game balance". What's the in-game-world reason?
Hell, what's the in-game-world reason marks trump each other?
In terms of the paladin, his mark is a supernatural ability. The reason the prior mark vanishes is because Tyr doesn't like to share the limelight with some fighter goon's taunts. It's a supernatural ability, so as far as we know a wizard did it. But suppose it's two fighters trading marks.
Joe Fighter marks the ogre, focusing on him and using a series of feints and challenges to get the brute in a position where any attention to any other PC will open up a big hole in the ogre's defenses.
Ed Fighter then comes up and does the exact same thing- except the sequence of feints and shoves that moves the ogre into position for _him_ puts him out of line for Joe.
Sue the Evil Paladin, working on the ogre side, then wants to use her mark ability on her ally to remove Ed's mark. The DM cuffs her for ignoring the spirit of the rules, and then tells her that Hextor isn't interested in helping a minion avoid a fight.
Willard the Evil Fighter, also working on the ogre side, also fails to get the picture and attempts to use his mark on the ogre to remove Ed's mark. The DM sets Willard's character sheet on fire after explaining that no, it doesn't work that way. In the roil of the _abstract combat round_, the ogre trying to deal with Ed who's really trying to kill him and Willard who is really trying to kill him (but not really trying after this round) and whom he shouldn't bludgeon while simultaneously treating his feints as serious threats yet not ignoring the _real_ threat of Ed... it's just not something somebody can process without results much worse than simply dealing with the first mark. If Willard really is turning coat and stabbing the ogre to death in mid-fight, then fine- it's a real threat that may gain additional oomph from surprise. Willard is no longer an ally trying to rules-lawyer some help for his ogrish friend, he's a brand-new enemy.
