ExploderWizard
Hero
It could work as a good way to balance spellcaster, or at least to show why fighters are always important, but it does break down.
In one AD&D campaign, through the luck of random treasure and goodies in published modules I had a magic user character with a 16 Dexterity, ring of protection +2, and (here's the key) bracers of AC 0.
AC 0? Was it a custom item? IIRC bracers didn't get better than AC 2 (plate and shield equivalent)
With my mighty AC -4 at level 9, I no longer feared losing spells. The campaign lost steam at about that time, as it became clear the other PCs were along for the ride on Kelgore the Mystic's adventures. It was frustrating for everyone, so the PCs retired after trashing Zhentil Keep and we fired up a new campaign.
Any campaign becoming all about 1 person at the expense of the rest of the group is doomed.
What it points to are what I think of as band aid mechanics. I don't like it when a class comes with a weakness (like a low AC) but the system provides plenty of ways around it.
It's a fine line. A wizard opting for better armor doesn't bug me as much as spells like mage armor. The former at least points to some interesting combinations and makes for some interesting character concepts, like a warrior-mage. The later just feels like an excuse to plug holes, especially at higher levels when you can afford to sacrifice a spell slot or a (theoretical) lower level utility spell.
I much prefer class-based abilities that let you mask a weakness for a moment. I really like the 4e version of shield because it lets you deflect one attack per encounter. You can handle the orc berserker for one round, but after that you need help.
I kind of agree about mage armor type spells. It was one of the many things from UA that just wasn't thought about enough. As for magic items, no system is immune from rampant Monty Haul syndrome, it's something the participants have to manage themselves to thier own taste.
I think the 4E shield is a bit weak for an encounter power. Something between the all time coverage of mage armor and the one round shield would be a good medium. Perhaps if the shield lasted 1 round every 2 levels it would be more useful, yet wouldn't be up all the time until higher levels.