I will quickly speak from experience here. I have been in a fist fight while carrying. I chose not to draw due to the circumstances and because it wasn't worth it. I got my jaw knocked - big fricking deal. He swung again and I locked him up and the situation quickly de-escalated from there.
More detail for those that care:
When the cops came and questioned everyone the officer asked about my knife in my pocket. I told them that I was carrying, the cop took my gun from me, and then asked why I didn't use it. I told them it wasn't worth the financial cost and all the pain and strife that would go along with killing someone in a bar fight (the other guy was drunk - I was in the bar but not drinking). The cop laughed and gave my gun back to me -- I overheard him telling his partner that I was carrying and his partner lecturing the other guy and his friends that they were lucky none of them were dead.
Everyone around told the cops that I kept trying to defuse the guy and that he took a cheap shot on me. The cop thanked me for my restraint and I went home.
Long and short it takes two people to actually fight.