True but the comment above gives the impression that marking is somehow quasi-realistic.
Opportunity attacks and such things don't make as much sense in an abstract combat system with somewhat long combat rounds. D&D combat isn't designed at its roots (static defenses, piles of hp) to be all that simulationist.
Dealing in all that detail yet not bothering to consider facing (locked in combat with a deadly foe then turning around to smack someone who runs past you doesn't give your original foe an opening?
) isn't very consistent in applying principles.
Detail is fine when it's flavorful, thematic and easy to determine. Facing is difficult because you have to mark all your minis, or pogs, or have an idea in your minds eye of where a character is facing, and then you have to determine how that actually applies to the battlefield. Determining whether a foe is or isn't in a character's field of vision from 30 feet away is SURE to lead to arguments between player and DM, gridmap or not.
Marking, on the other hand, if flavorful and thematic for the fighter. It's the fighter, through his actions, saying "you're my target; I DARE you to ignore me." It's fun, it allows the fighter to protect the squishies, and it significantly increases damage output if the monster ignores the mark.
D&D is an abstract, non-simulationist system, true, but picking the level of abstraction is important - and making sure where you draw the line is fun and thematic for the character type is important. Thus why marking is a good thing to add and facing is not.
I never understand why people are so against marking. It lets a tank be a tank and at no point is it unrealistic or breaking of verisimilitude. Half the time it seems to me like the actual argument against marks is "because its from 4e and 4e is bad."
This line of conversation reminds me of a post I saw not too long ago from someone either here or on rpg.net that explained marks using pictures from Basketball and another with a mother polar bear standing defensively over a baby polar bear that said something like, "go ahead and attack the baby bear. Guess what will happen." Maybe someone will post it, because really, its the best justification for marks I've seen.