I remember, there was a passage, that stated, that you can't just add aspects to a defense roll as you like, once you beat the attack roll, that is it. This was about creating spin or something, but I would apply it here as well. That might mean, that you only wear down your opponent 1 stress at a time, but he won't be able to get close to you, without gettimg creative. Remember, you don't even get the bonus for full defence, if you riposte an attack. There is nothing to say against a defensive fighting style, though. It might even get compelled, when you can't afford to wait for your opponent to act, if time is important.