dmgorgon
Explorer
First, why am I now exerting myself to dodge his blows? Why am I graceful and quick yet continuously messing up my positioning (and remeber I am also a skilled warrior/rogue/whatever). Why isn't he exerting himself more to actually land those blows on someone as graceful and quick as me? This is what I mean by one narrative being subsumed by the other.
Mechanically I am being whittled away towards the conclusion of me either being hit by his weapon and being taken out or me being missed by his weapon and still being taken out... mechanically I am not dodging anything. In other words even though I am (clumsily, which again messes with my narrative) dodging his blows in the fiction... the fact that I am always taking damage doesn't mechanically support that fiction... especially since at least part of hit points are meat, blood, etc.
You are correct, narratively the opponent defending against a rapier should have to move a lot quicker and dodge far more often than against a slow brute with a great maul.
Mechanically, the game does not care if you take cover behind a portcullis or hinder your foe, there is nothing you can do to avoid being "taxed".