Well your argument is that it has nothing to do with magical properties, that it's just that putting on a bracer innately negates a shield regardless of if there is any magic involved or not. This would mean that putting on a non magical bracer would negate your shield just as much as putting a magical bracer on would.
Benefitting from 2 items at once is irrelevent, because by either interpetation of the rules you could only benefit from one magical property, and the bracer propoerties are not obviously better then the shield ones.
My argument is nothing of the sort. My argument is that you can only use one item in any slot at a time, my argument is not that using bracers negates the shield. You are perfectly able to use the shield, OR use the bracers. But you cannot use the shield AND the bracers. To change from using the shield to the bracers you need to take the shield off and put the bracers on first. To change to using the shield you need to put the shield on first. As per RAW
Now, the other part of this is that non-magical bracers give no benefit, and so it wouldn't matter if you put them on or not, you would still qualify as "receiving no benefit from the item" and so could still use the shield since you were only receiving benefit from a single item. Why could use use the shield? Because you are receiving no benefit from the non-magical bracers. Its the same as saying that you put a feather in your cap. Mechanically it makes no difference at all.
Mechanically it DOES make a difference, because people cannot use bracers and a shield at the same time it limits shield users to magical shields. It means they cannot get both the personal protection of bracers of defense(mechanically superior to all self shield protection for the most part), and the +1-3 AC from a shield.
It means that people in heavy armor cannot gain their int bonus to defense(otherwise they just wear cloth, are now wearing light armor, and so add their int/dex bonus to defense), or benefit from boots of striding and springing(same as above)
It means that people using light shields cannot gain two weapon fighting benefits.
It means that you have to make trade offs for that mundane or magical item you want to strap to yourself just because
edit: Bucklers died in in the conversion from 3.5 to 4e for a reason.