While in reality your point about "10% of the time" isn't always true when penalties are applied, because with a -2 penalty - 55% chance of hitting now compared to 65% before - you end up with only a 30% chance of getting the automatic damage. A 12% difference compared to without the mark, which as you can see actually increases the effect of the penalty.
In fact multiple attacks suffer most from marks (like solos), especially if they are one of the monsters that needs 2 or more hits on a single target for a subsequent effect.
The 10% does still apply ... it just increases in value in certain situations. Every time the enemy attacks, the penalty applies again. So 10% to each roll, when the creature rolls twice is going to be a big improvement, even more so when something relies on both attacks hitting, etc.
The aura vs. marking situation definitely slides towards the marking being better when elites and solos are involved. Multiple attacks means the penalty applies multiple times. In the case of, for example, a hydra with 5 heads ... marking it would (alone) be responsible for one miss in every 10 attacks, which is only 2 turns, or one turn with an action point, for the hydra).
Shield works 30% of the time with a mark. It works 40% of the time with a mark + enfeebling strike. It works 50% of the time with a mark + enfeebling strike + illusory ambush. Heck a good chunk of the time you wouldn't need it in the first place and saving a valuable encounter power, because of the handy -2 penalty (that freely stacks with a bunch of other at-will penalties) is better than nothing.
Yes, -2 is better than nothing, if that is the only argument. However, in some cases, there may be a choice between marking multiple enemies, or marking one but perhaps imposing some other condition on that one enemy, etc. If you the choice is LITERALLY between a mark and nothing at all, than there is no reason to pick nothing at all. But people don't grab every defense bonus they can out of feats, or nothing put powers that impose attack penalties, etc ... because you have to compare it to what othre options you have.
And, ultimately, if you stack 3 different penalties and buffs together, they do combine to make an attack nearly impossible ... but EACH of them:
(a) still works without the other ones being added
(b) each only contributes a small part.
Example. Enemy needs a 9 to hit the wizard. You have mark, enfeebling strike and illusary ambush.
On a roll of 9 or 10, you only needed one of those to make it miss. On a rol of 11 or 12, you needed two. On a roll of 13 or 14, you needed all three. On a roll of 15 through 18, you could use shield to stop it. So, all those effects together did make it nigh impossible to be hit ... but, aside from the shield, each individual debuff only contributed a small ammount. A roll of 1 to 12 or 15-16, 19-20 would have all been exactly the same even if the mark was removed from the equation. 13 or 14 would have meant you needed to use shield when you wouldn't have had to otherise, and 17/18 wouldn't have been able to use shield to make it a miss ...
Is marking good? Yes. But a mark isn't responsible for all the other stacking penalties. It is a small (albeit reliable) part of a bigger whole.
They work far better when you have a free and easily acquired -2 penalty on top of them. Especially when that penalty has another penalty on it and such forth. In my second epic tier game, the Sorcerer had an ability that added a -2 penalty onto any creature he hit (which was pretty good in itself).
Yes. It was pretty good in and of itself. It gets better with a mark, but that doesn't actually make the mark better. They are both good things that happen to be good together because they do the same thing. But 10% + 15% doesn't end up being MORE than 25% ... they both do the same thing they did before, they are just allowed to stack.
Of course it is! Are you reading my arguments or what? Taking optimal choices is optimizing. I mean that is pretty self evident!
So you need to optimize a knight for it to be competitive compared to an optimized fighter. Fine. It seemed that before the whole "but you have to optimize the knight" argument was that knights need to optimize just to "catch up" to unoptimized defender.
When they are immensely important to how a Knight functions, yes there is a big difference. I mean a non-dwarf Knight who lacks defend the line is just sad. There is a good reason that dwarf is one of the best rated races for the Knight (minding any defender, but Knight especially) and Defend the Line is a pure gold stance.
Even I will concede that a Dwarf Knight, with Defend the Line and World Serpents Grasp is actually a pretty effective defender. Once in paragon where the struggle begins again, just take a PP with an actual marking mechanic in it and you're done. Now you've got an amazing and easy to use marking aura, with a "real mark" for anything important or that the aura has difficulty with.
But this is *pure* optimization as the gulf in effectiveness between the builds is massive (almost a gaping canyon IME). You are right that a fighter not taking combat challenge or the -3 isn't the same thing. A weaponmaster who doesn't take those isn't basically curtailing their actual viability and they never need to take the -3 feat (though they probably will anyway). You may feel a Knight without those things is fine, but in my actual play experience without those the Knight struggles immensely.
A knight without defend the line is, in my opinion, like a weaponmaster that takes combat agility. The ability to chase the enemy and knock it prone is just so much weaker than the alernative, because you are basically abandoning any other enemies you had marked, not to mention the people you want to lock down the most are the ones you can't follow (like the flyers).
Lacking the forced move protection from the dwarf or the prone trigger from World's Serpent, a knight with defend the line should be ok. Slowing enemies covers what, IMHO, is the most important thing, which is being able to limit the opponent's access to allies ... a slow enemy is comparable to an enemy that needs to shift to get to another target ... they get one extra square of movement, but they may even lose their chance to charge ... especially if difficult terrain is also involved.
When you add the prone effect from World's Serpent, I'd argue that there are at least a good number of situations where the knight is more useful. A prone enemy that already used it's move either has to waste a move to stand, or get a -2 penalty to attack, not to mention having to attack from the current square which is probably not the square they were trying to move to ... and they'd still be using up another move action to stand later.
The needing to multiclass just to grab a secondary marking mechanic via a paragon path is the kind of optimization that is a bit much, and is the big thing I'm arguing against. Ditto the need for a knight. Comaring a slowing/proning knight to a stopping on an OA fighter ... when they are up against forced movement, the ONLY penalty left is the mark penalty. Which is good, but I'd argue isn't as important as you put it. In a group that is focused on maximizing penalties to attack rolls ... yes, taking every single attack roll penalization is a good idea because it maximizes those effects. Interupt powers that boost defense, anything that forces rerolls, enemies that attack multiple times, etc ... each causes the mark to have more chances of happening in any given fight.
Long story short ... the knight needs to be put into a situation frequently enough to make taking that paragon path worth it. Going with a -2 mark, there is one attack roll in every 10 where the mark will be the difference between a hit and a miss (powers like shield do double the chance of making a difference when the character has it available), so the person considering grabbing the marking ability needs to figure out how frequently they will be in a situation where:
(a) They would have applied this new mark
(b) The enemy would get out of the aura
(c) The enemy would get an attack off against an ally.
Let's say this happens twice every encounter. You would cause a near hit to be a miss instead once every 5 encounters. Even with the effective doubling from interupt powers of allies ... it's once every 2.5 encounters. Compared to some other features you could have taken instead ... some people may prefer something that will occur more often.
Again, it depends on how often it will come up. Your encounters apparently tend to have that scenario happen often enough where World's Serpent and Defend the Line just aren't enough (and the party does have power selection which make any penalty to attack roll better, such as shield and staff defense). However I don't think that's typical of everyone's games.
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