The problem is mark penalties can have
other penalties frequently stacked on them. A -2 penalty that follows you around is extremely difficult to deal with on top of other penalties imposed as well. Additionally, something is always absolutely better than nothing and if I told you "Here is a way for me to give you a free +2 to all defenses" would you say no?
The fighter's mark punishment is every bit as easy to evade as the Fighter's, /and/ the fighter's is strictly 1/round, and is a vanilla MBA, while the Knight's is stance-enhanced, and does damage on a miss.
While true, the Knight will rarely get equivalent to the fighter because monsters don't need to fear their OAs and can move away. A fighter has damn scary OAs because you cannot move away
and their accuracy is boosted. So while the fighter will usually only get his immediate interrupt punishment once, he never concedes OAs as it's simple suicide for a monster to attempt to move. A knight who isn't using defend the line isn't as accurate and has no threat beyond just an MBA whatsoever - creatures can just depart and hit whatever they want.
Also a knight doesn't do any damage on a missed OA - which is what monsters will provoke and NOT their battle aura (why shift and provoke battle guardian when you can provoke a regular OA and ignore the Knight anyway?). This is why of course I say Defend the Line is an
absolute must for the Knight. Defend the Line slows enemies on a hit and is essential for preventing enemies just walking away. Without it, the Knight can severely struggle and it's why the best Knights I've seen use Defend the Line + World Serpents Grasp - it just fits! Defend the Line and the knocking prone of WSG gives the Knight a lot of teeth - but without it they are practically toothless.
Whereas a fighter moves into difficult terrain (where enemies couldn't shift) and uses come and get it.
Good luck leaving. On the other hand, if your Knight somehow gets into that position (difficult in itself, because you can't just pick it out ala the Fighter) and you made the mistake of not taking defend the line, monsters in that scenario are nowhere near as punished - they can leave pretty easily.
Same goes for the fighter marking at range with barely-effectual heavy thrown weapons. It's better than twiddling his thumbs, but not really up to snuff.
-2 penalties that are stacked with other -2 penalties, especially as this
doesn't ever need to hit to be imposed is pretty amazing actually. I mean, would you rather be facing a dragon you can't reach with at-will "Impose a -2 penalty on all this creatures attacks that don't include you, perhaps combined with the controllers at-will penalties?" or "Twiddle your thumbs and hope for the best?".
There isn't really an option there
