But then you lack narrative consistency. My character, with his lightning quickness and magic rapier, is capable of knocking aside crossbow quarrels with a twist of his wrist - unless they're poisoned.
Where's the inconsistency? "Capable of" does not imply "never fails to".
Your PC was, and remains, capable of knocking aside those quarrels. But on this occassion s/he didn't, and hence got poisoned.
Now if very many arrows or quarrels in the game are poisoned,
and your swashbuckler PC is routinely targeted by them,
and the GM is not rolling very many misses with those attacks,
then the narrative might start to look a bit wonky - the facts of play no longer bear out your account of your PC as a deft deflector of arrows.
In my own game that problem doesn't arise, because not many arrows or quarrels are poisoned, and lately I'm having trouble breaking 10 on my attack rolls, and so have plenty of misses to my credit.
But I can see that it might be an issue in (for example) an assassin-oriented game.
A similar issue (similar in structure, that is) arises with (pre-errata) Come and Get it. The PC in my agme with that power is a polearm fighter. He has heaps of abilities that pull and push and slide one or more enemies. When he uses Come and Get It, the narrative is generally not one of taunting, but one of deft work with his polearm that wrongfoots and ensnares his foes.
Now if he were using a dagger rather than a polearm,
and/or was frequently fighting on terrain which made the idea of deft footwork implausible, then we might need to find a new standard narration.
This is why I think I agree with [MENTION=386]LostSoul[/MENTION] that abstraction, with the actual players at the table filling in the details as makes sense for their fictional context, is a virtue rather than a flaw. But I think it would help for the rulebooks to talk a bit about different ways of handling these things. Particularly around some obvious and notorious difficult areas, like falling damage and PCs caught in the centre of fireballs and dragon's breath. (I think the AD&D DMG actually did a better job on this particular issues - especially saving throws - than the more modern rulebooks. 4e's rules
needed this sort of commentary/guidance, and suffered from the lack of it.)