Simply doing dungeons faster than you is game breaking? Wouldn't that technically mean that anyone stronger than you are also game breaking? Food for thought.
1. A weekly contest that be completed a multitude of ways does not mean a single skill should be nerfed. A simple way is get a mount, go to Novice, and complete dungeons with hardly anyone around. You are not required to use NN if you don't want to; there are far more ways to do this contest that are even more efficient than what I said without needing the class.
2. Just because someone is going faster than you does not equate to trolling. They could have been there before the challenge and just be playing. They might want to avoid the lag that the leaches bring to challenges so they avoid groups. Maybe they have limited time and want to finish the challenge but the group is slow. Simply calling someone a troll without knowing the full story is not helpful to anyone. On that note it is also best to stop calling anyone who opposes your opinions a troll as well; won't get you very far here.
3. They always considered it an exploit and we all knew it was getting removed at some point. While it was used to speed through challenges, that was not the reason for its removal. It was removed simply because it was the very definition of an exploit which is using a feature or item in a manner that was not intended by the developers often for personal gain.
4. NN flip is still not an exploit no matter how many times you call it that. Imagine calling CB's Sour Candy an exploit simply because you also happen to use AS gear. Sounds funny right? Well that is the exact same situation with NN. Some people simply like to go fast while others like to go a bit slower (take me for example); punishing them just because it does not fit into how you think the game should be played is foolish.