Well it depends on the situation. I see three.
1. Sparring- If you are good at martial arts and have a friend who is as well, sparring with him or her can make your skills better and hone both your abilities. This sort of fighting i think can be very beneficial to your physique and technique.
2. Revenge- We've all seen it. One guy kicking the crap out of another guy for messing around with his girl, or because on man wronged another in a similar manner. This i view as acceptable, but only when negotiations have failed.
3. Instigation- One person picking a fight for no other reason than sick amusement. This i find to be simply unacceptable. Fights should have a good reason or should not happen at all.
I know these are not the only ones out there, but every fight should have a reason. In the animal kingdom, all fights have a purpose. In our society, though, often times fights have no reasons and mostly end badly.
So, overall, my view on fighting, or violence in general, is that if there is a valid reason for the violence, let it run its course. If not, stop the fight.