Remathilis
Legend
That's a bit like claiming that chain mail (part of the starting gear for fighters) is as powerful as Bracers of Defense. I'm not seeing it. And it's quite odd that you consider nonmagical plate armor to be rarer and more expensive than mithral/adamintine plate armor. I think you are inferring the wrong things from the 500 gp "construction price" for uncommon magic items. But, YMMV.
The difference being there is no other way to get an 18 without a shield or medium-armor mastery. Yeah, its a flimsy, but if you have to justify its high price tag...
In truth, cost aside its probably worth a common or weak uncommon item.