I think the Devs oversimplified an old idea of Offense/Defense and tried to implement that, unintentionally bypassing realism in this case. Subsequently light armor became a symbol for defense, while sword, strength were its counterpart on offense. Unfortunately training defense meant training light armor which makes little to no sense (as its been previously discussed on this thread).
The quality of the armor doesn't change with time nor does active use improve the intrinsic strength of the armor. It makes far more sense that the person becomes more agile within the use a particular armor, or quicker with use, and I think that would be taken care of with the stat "Agility." However agility with each type of armor is slightly more difficult, and because they aren't the same, each armor must have its own particular technique. The stat should be more appropriately titled "light armor agility" but I'd argue that it'd be more of a function of experience and not training. Its quite possible someone in would don some chain mail and actively practice it in a room, getting better with time. I just don't see it. I'd also be hardpressed to argue that this is mutually exclusive of "Agility" itself.
I'm sure the Devs will figure it out and remove light armor in the future. It seems easier to categorize defense by a few stats, namely light armor, and then just level it up. Perhaps, I can only assume, for playability and not realism.