--If they miss an attack with a roll sufficient to hit AC 33, they will figure out who not to attack again.
--If one PC is springing back and forth, dishing out a whopping 9 points of damage while the barbarian up front is dishing out 50, it is pretty obvious who needs to be taken care of first.
On top of the obvious fact that AC 33 would be a very relative number (obviously at higher levels it's a piteously low roll), there are two things to take into account:
1) Only an INTELLIGENT creature would deduce this. Some animals, vermin, undead, constructs, etc. certainly would not. How the hell would a Golem reason like that?? Or a Gelatinous Cube???
2) What you believe to be a telling blow might just miss (as in your example); would you then abandon the attack entirely? What if you were facing an entire buffed-up party? Past a certain level, you certainly will. Do you just stop attacking and let them kill you because you can't hit them and they surround you?
Now as for your second example, again you're exaggerating. By the time the Barbarian does 50 points of damage per round steadily, the monk can do so as well. Consider a Half-Orc Fighter with a Greataxe, and a Str of 24 (with items). With Weapon Spec, and let's say a +3 flaming axe, he deals 1d12 + 15 + 1d6. He needs at least two attacks to do 50 points, but for that equipment he should be higher than just level 6. So let's make him level 12. He can then quite conceivably deal 75 points of damage a round, more with Power Attack. But he's probably wearing Heavy Armour, bringing his speed down to 20'. Still, he attacks at +23/+18/+13, mostly likely easily hitting at least with the first two. Not many creatures have ACs that high at a CR 12.
Now, the Human monk (let's say), with a Dex of 22 (again, with items). He now deals 1d12 damage with (let's give him a 12 Str) a +1 damage. Now that's not much, especially with the magical axe, so let's give him an equivalent item, +3 Flaming Fists of Fury (which are special ninja gloves--not gauntlets!--with a flaming enchantment). There's still a +11 damage difference. With Weapon Finesse, this monk attacks at +18/+15/+12 (with a possible +1 for Weapon Focus if he's taken it). Say he flurries, he then attacks at +16/+16/+13/+10, which is perfectly respectable, dealing approximately 14-15 points of damage per hit. That's acceptable, and he'll most likely hit at least twice per round.
If the monk had had 22 Str instead of Dex (a probable but not altogether attractive scenario IMO), he would deal about 20 points per attack instead, not far from the fighter's 25. The monk is then a tolerable damage-dealer (certainly better than most clerics!), only falling truly short in terms of critical hits. There's no reason he has to equal the fighter. But he's not far. When they hit level 16, the monk is much closer still.
If the monk had Monk Tattoos, from Magic of Faerûn, he'd do 1d20+9+1d6 damage at level 12 compared to the fighter's 1d12+15+1d6. Not bad after all. Granted, he may be a little over-budget (not sure), but it was only a hypothesis.
Now on top of this quite acceptable fighting ability, the monk moves 3.5 times the speed of the fighter, can heal himself spontaneously if needed, is immune to poison, and can Dimension Door out of a tight spot--or INTO one, to help friends. If one of the characters was hard-pressed by a foe, and the other started 70' out (a possible scenario), the monk would be there the same round with an attack to spare, while the fighter would have to all-out run, and THEN walk 10 more feet.
So yeah, your fighter deals more damage--but that's all he can do. In that armour, he drowns if he falls in water, while even the most hydrophobic monk stays afloat (ok, maybe not, but you get my drift). If your fighter is swallowed, chances are his Greataxe won't help; the monk's backup +1 kama is fine, though.
Just to point out that the damage difference doesn't have to be so large, and the monk can handle himself quite well otherwise. "Pulling your own weight" can mean a lot of things, and often it only involves surviving to help your friends. You can't help them fight if you're Held, Dominated, or dead. And the fighter better hope that the Dragon who breathes fire on him is young, otherwise he's a walking s'more
