He will have, at level 12 assuming a +3(Githzeri) Plate an AC of 10[base]+6[lvl]+8[plate]+2[gith]+3[enh]+1[spec]+1[pit fighter]+2[shield]= 33 AC.
Now lets take a level 12 Wizard, 20+3 int, wand. He has an AC of 10[base]+6[lvl]+6[int]+3[enh]+1[special cloth]= 26
So if an enemy decides to attack the wizard as opposed to the fighter, he will take in an average of 8.2 damage each time he does so and his DPR increases by 5 damage per round.
The wizard has ~68 hitpoints. The fighter has ~133 hitpoints. That carries over to how much they heal when they spend surges. All that adds up to mean that every point of damage that the foe does to the wizard effectively counts for twice as much as if he'd done it to the fighter, because it comes that much closer to downing a foe, and uses up that much more in terms of healing.
So, the monsters DPR adds 5 and then effectively doubles. That's a lot more than the 8.2 damage that he's taking back.
This does not need to occur. The Fighter could have 8 surges left over and the rest of the party can have 0 to 2 surges left over and it could still be good. There is no way that a party can balanced this out completely, so it makes sense to have the PC attacked the most be the most heavily defended. If the Fighter never or rarely uses a surge in a given day, it just means that the Leader can spread his healing across 4 party members instead of 5. That's actually a good thing.
The fighter using surges or not has little to no bearing on how much the leader heals: leader heals are restricted per encounter. The daily resource here is surges (with some powers being notable exceptions). If the fighter is ending on 8 surges while the rest of the party is on 0 to 2, then assuming each extra surge worth of damage he takes is equivalent to removing a surge worth of damage from another character (which isn't true: defender surges tend to be larger than those of others), the party could have gone another encounter before running out.
If foes ignore the Fighter, they will take a beating.
Also, even if the Fighter has an AC of 6 higher than the Wizard (unusual in the game, but it could happen), the DM would have to metagame the foes to have them take a swing from the Fighter in order to take a 4 better swing on the Wizard.
How does a foe KNOW that the Fighter has AC 3 higher than the Wizard without the DM metagaming the info?
A Fighter is not a Striker. Having a decent attack is good enough. It's not as if that's difficult with little effort.
The fighter's counterattack is usually only enough to make the decision between low-hp and high hp targets even, assuming close defenses. Once the defenses start to shift, you need to make up for that AS WELL.
As the fighter's damage dealing ability goes down, he not only becomes less effective at counterattacking: his value as a target goes down as well. So not only does the wizard have less hitpoints, but each hitpoint holds more value.
As a totally non-muddy example: we have two combatants. One has 5000 hitpoints and does 1 point of damage per attack. The other has 1 hitpoint and deals 5000 points of damage per attack. Which do you kill first to maximise your chances of winning?
Wizards jump out of the way of attacks nearly as easily as those tanking Fighters glance an attack off of their armor. That's the physics of the game world.
So, NPCs should think that Wizards are not necessarily much easier to hit. Unless of course the DM is metagaming the exact numbers.
Personally we and our DMs reveal defenses when you attack them. It just makes the game flow that much quicker.
Even if you don't do that though, defenses get pinned down pretty quickly. And if you're talking about large differences (ie - the sort that will result from prioritising defense on a fighter and giving him all the best defensive gear vs prioritising other roles) then is it really metagaming?
The point is, is it worth a -2 to attack the squishier targets when there is also a fair chance that the Fighter is also going to get a free hit on you?
Usually it comes pretty close, but a good deal depends on how hard the fighter actually hits.