Or to make the monster less effective.
The monster not attacking you is being fully effective, because he's
not attacking you. Especially because your shielding aegis will do nothing to effects, auras, automatic damage and similar. Many monsters increase their DPR a lot by these mechanisms, so preventing a small amount of one or two (at most) attacks doesn't really bother them. You have 3 marked, but they know you can do very little about them just hitting your allies. A Marilith, for example, attacks six times just by herself. The Herald of Colorless fire gets 2 burst attacks and a shift in between (noting that the bursts won't target the same PC twice, so one will be subject to a mark).
Point is, the Swordmage isn't preventing anything in reality because the monsters aren't attacking him. He is failing in the role of defender because he's failing entirely to draw a monsters attentions.
Even if it's 75% more, that's 67 damage. If 27 of this is blocked that's 40% less damage.
For one attack. Now what about his second? Third? Fourth?
If I kill him on attack 1, then his second attacks damage? 0. Third? 0. Fourth? 0.
That's when it matters. That's also when the monster is
forced to engage the defender. That means he has to engage your high defenses, lots of HP and healing surges. This is what the swordmage
utterly fails to achieve: Being noticed.
And you are missing the overall math.
If the monster lives 6 rounds while losing 40% of it's DPR
40% of its DPR? Have you looked at epic monsters these days? They do far more than 1 attacks worth of DPR. In addition, you have forgot in this post that much of that DPR is boosted by aspects your shielding are unable to effect: The monsters aura, ongoing damage, automatic damage effects and similar.
Additionally if you're a fighter and you hit a monster two rounds in a row ignoring your mark, he loses an entire turn of extra DPR (because he'll be dead). That extra turn the creature gets because in the same situation he isn't being damaged, makes all the difference. Because PCs by epic are really pulling some serious AoE and similar damage as well. By round 3 or so, there isn't much of a chance that most monsters on the battlefield (if they aren't dead) aren't badly damaged already. In fact it's usually the case that monsters at epic tier, due to the power that PCs have, can't live long enough to do much anyway (which is why encounter design and such is so important - but this is another discussion).
to the swordmage opposed to living 4 rounds due to being constantly hit by combat challenge, the monster will have done less damage with the swordmage.
With the exception of the Swordmage, even an epic monster doesn't get 4 extra rounds of being hit by combat challenge. Noting that I will concede that can happen with a solo, but a solo has the HP to tank the combat challenge. Even high level skirmishers or similar can't. 4 combat challenges is around 160 or so damage - that's a fair chunk of HP. Not to mention a Fighter/Warpriest not only gets CC, he get's an OA so attacks twice every time his mark is violated.
6 rounds of 60% damage is less than 4 rounds of doing 100% damage.
Of which at least 1 or 2 rounds is dedicated entirely to attacking the fighter. All that damage that the swordmage is "preventing" as you claim is actually being directed against your allies who don't have the HP/surges that you do.
Also since the fighter needs to be adjacent, it's likely that the monster just hits the fighter instead
You realize this is the entire function and point of defenders: To be hit instead of their allies. The shielding swordmage is failing because the enemies are beating the snot out of his allies and ignoring him.
Against which the fighter is just as helpless. More than 2 (with the right epic feat) he can't punish either
The shielding swordmage suffers far more than the fighter does. The fighter is at at least
making enemies attack him, while the shielding swordmage is failing to accomplish that at all. Bear in mind, that when the Wizard and Cleric are on 20 HP, that monster you've got Aegis'ed is not worried about you reducing his damage when he punches them unconscious. On the other hand, when it's a case of "Do I attack the ally and potentially
die doing zero, though I might get my aura that does 10 damage" or "Do I attack the ally and just lose a bit of my already high base damage, especially as I'm doing 15 ongoing with that attack and have an aura that does another 10 damage"?
There really isn't a decision there and that is again my point: There is no incentive to attack the shielding swordmage.
Mirtek said:
Now keep in mind that no monster will be doing the best limited damage expression every round and since the fighter must be adjacent might decide to simply hit the fighter instead (making the fighter do nothing toward reducing the damage the party takes during that round).
You edited this in, but this is absolutely wrong. This fails defender 101. The Fighter being attacked
is reducing the damage the party takes. He's making a monster attack him on high, rock solid defenses or die - increasing the chance of missing and doing absolutely zero damage. Even if he hits, the Fighter is tanking that damage through higher HP and surges. You avoid this part of the argument deliberately, but the swordmages reduced damage doesn't matter when the attacked PC has less HP, surges and defenses anyway (and again, that much of the damage that monster will be dealing can't be reduced anyway, like ongoing damage riders).