A single piece of metal like a bracer, doesn't constitute being decked out in plate armour. We're not talking a set of leggings or something like that. I don't see an RP issue in that.
What you MAY see is a code issue. That bracer with all your other armour may pop you over the threshold of your armour proficiency. Since we can piecemeal and layer armour, the code totals all this up.
You will know, if you start getting a bunch of "your armour hinders your attempt" messages. Then you have to decide whether the bracer is worth the feat point to train up your proficiency.
EDIT: This is to assume we're talking the RP of a ranger, as far as the bracer goes. For a druid, even a simple metal bracer would be a definite NO.
