It doesn't even have to be that, really. It's simply what the fighter (or any other defender) is good at. The enemy doesn't have to care - it's simply a factor in their decision-making.
In the case you mentioned above - a high-level opponent most likely won't worry about the fighter's mark. That's a direct result of the fighter's marking and combat challenge mechanics - if the fighter is having trouble hitting, and is dealing negligible damage, then a high-level foe can and should ignore them completely.
-O
Yeah, that's true. Remember, Defenders don't need the enemy to respect their mark in order to function...they can sometimes be just as happy if they're disrespected.
Note the quote in my sig from the "What class are you?" thing. It says "You'l have to deal with me first Dragon!", and that's what Fighters do. In fact, in our last session we actually fought a Purple Dragon, and I did exactly that!
In the beginning of the encounter the Dragon had a shot with his breath weapon and thought he could get more enemies in the blast by ignoring me (basically, including me in the blast would've left out two allies)...which was his first mistake. I whacked him upside with my Execution Axe for some nasty damage to remind where he should stay focused!
Later, when he ran out of powers and all he had was bites and claws, he mainly used them on me. Not so much because of an RP thing where I was "distracting him", but more out of the very real fear that ignoring would result in another smack upside the head. Eventually, he tried to get away with a move action (there were other melee allies around him, so a shift + move wasn't an option to avoid getting hit by me...in fact, it could've been worse that way because I would've gotten a CC
and an OA!) so that he could fly away and harass us from the air. So, again, I smacked him upside the head with my Execution Axe for some nasty damage and stopped him dead in his tracks. He could've blown
another action to actually move away, but at that point he was bloodied and practically keeling over, and I guess didn't want to risk even
more OA's from the rest of the party, not to mention blowing his whole turn.
So, as you can see, my mark was ignored multiple times in that encounter. It was no big deal though, and a helluva lot of fun, because I got to tack on the extra damage. I think all told, I probably did about an extra 40-50 damage (out of his 340HP total) due to him violating my mark and provoking an OA. For other Defenders, it can be even more fun. Auto-damage for the Pally, teleporting or damage mitigation for the Swordmage, and extra attacks or slides for the Warden...all of these can really add to the firepower of a Defender and make for exciting encounters.
The only time a DM ignoring your marks universally becomes a BIG issue is that if he says that there's no RP reason for the enemy to get a -2 on attacks against your allies, and therefore you can't have it. If that's the case, then that's pretty crappy, and I would expect the DM to come up with a valid RP mechanic for everything else in the game, such as the bonus to saves for Elites and Solos, which he probably can't, because being "tough" applies equally well to the PC's. In which case I would argue that Elites/Solos just being "better" at it is the same argument for why Fighters get to mark...at least IMO.