I can agree, to a point. This is a semi realistic game, not totally, but a little bit, anyway. I don't think you should be able to get away with totally concealing an armor that makes you quite a lot tougher in the way combat or metal armor does, it just doesn't figure. Our better forms of armor are quite bulky at times, and I can't see easily concealing some of them. If nothing else, your better forms of armor would make for a bulkier looking robe model, but without actually suggesting whether it's metal, or combat armor, or combat armor mark 2. Your lighter forms of armor might reasonably be concealed. Ultima Online typically allows robes over armor, but you usually get some hints, like seeing a helmet, or noticing pieces of armor, or subtle clipping issues with the models, or other little things.
I do like the idea of disguising yourself though, within limits. You could even bluff people by only having a robe, or a robe and a jacket, and leave people wondering: is that guy naked under that lousy robe, or does he , for instance, have a combat leather jacket, or leather armor? Or whatever.