I thought that it was actually illegal to wear any unnecessary body armour at the plate. I think it may be one of those things that the umps look the other way on. Like hitters with one foot outside the back of the batter's box.
I think that's another trend much like the pants pulled down over the heel of the shoe...that Barroid is responsible for.
I remember after Bagwell kept getting plunked in the '90's he actually got special permission from the league to wear his armor.