See that isn't metagaming. Creatures above animal intelligence should be able to tell they are having no effect upon a target and switching to a different one is perfectly reasonable.
I believe that ANYONE can tell if it's attacks are been ineffective.
Animals, Oozes, Constructs, anything.
These are extremely basic capabilities that any combatant should have.
To be a threat, that is.
This is a common fallacy.
I don't think it's a fallacy, not in the strict term at least.
I you were 100% sure that you would force an enemy to attack you, then of course pumping your defenses would be good thing to do.
If not, then there will always be situations where a high defense will make an enemy choose the other squishier members of the party.
The trivial situation is when an enemy can choose between moving adjacent to you and attacking, or moving adjacent to a striker and attack him.
Where it pays minimal or no cost at all into choosing the easier target.
A middle case is when he's adjacent to the defender and marked/challenged/aegis/etc.
Defenders should have the best AC they can get. They should then take powers like Come and Get It, and Feats like Daunting Challenge, and Mark as many foes as possible, and try to keep as many foes attacking them as possible.
But note that they can't really force them to do that.
They can only force them to a Catch-22 deal: hit a highly durable foe, or suffer damage/conditions for hitting a softer target.
So a trade-off WILL be made.
And this is where an extremely high AC can result in a problem.
Imagine that your Defender can only be hit on a 20.
No one would EVER choose to hit him.
Of course, all what I'm saying depends heavily on when do enemies know the AC of the party members, or at least who's softer and who's harder.
Which, I think, it's the real issue in your argument, and a very important one in the effectiveness of defenders.
On the matter of who's softer and who's harder, I assume that the enemies know this at first glance. Obviously some exceptions exist, but I'm talking in general.
This is as fair as when you, as DM, describe the enemies and drop hints on which enemies are tougher and which are softer, which are mobile, ranged, etc.
I assume you DO tell the players this stuff.
The foe should not know the AC of any PC. If they do, then the DM is metagaming.
Ok, now onto the meat of the argument.
Unfortunately, the game doesn't specify how to manage information at the table.
Worst of all, it doesn't state which parts of the rules assume what information should be handed out and which should be hidden.
In any case, this is the first edition that explicitly says that some things MUST be told: bloodied, effect of conditions and in some Dragon/Dungeon articles, which are minions, etc.
Note that the game works whether you know the opponent's defenses or not.
But if you have information, you can make better choices, and a lot of interesting stuff happens. The game becomes much more richer.
To give a quick example: if you don't know that two enemies gain AC when adjacent, you'll never use your forced movement powers to aid a melee combatant that's attacking them.
It's a lot more fun and interesting if the opponents know of this feature.
I tell the defense score of a target when you have line of sight to the attacker and the target when an attack is made.
This applies to both sides.
I normally don't track what the enemies know, because I usually assume that a Defender is an obvious one, and the monsters don't go for them (most of them at least).
But in some cases, mostly to not be unfair to the players, I track the enemies knowledge and act accordingly.
The secret is to have an algorithm or methodology decided previously on how they act. So you don't metagame and you free yourself of thinking what they should do next.