If you want to enter an MMA tournament, then it makes sense to know ground fighting, submissions, clinch technique, takedown defense, and some form of boxing or kickboxing.
These same martial arts might help you against someone on the street if they are dumb enough to let you know they're going to attack you and come at you in the same manner as a boxer, a wrestler, or a mixed martial artist would come at you in match with rules.
Otherwise, street fighting is hard to predict. It's better to train in almost any martial art than to be completely defenseless if someone is going to attack you, but a good street fighter is probably not going to want to get hurt and he will make sure that you will not be able to use physical attributes or fighting skills to your advantage.
For every idiot that steps to you, engages you in a stand off, or otherwise gives you the opportunity to attack first or otherwise prepare for whatever it is he is going to come at you with, there is a smarter man who is going to make sure that when he attacks, he'll have every advantage. If someone comes at you from behind with a weapon in a room full of his friends and you happen to be drunk, then your martial art better have prepared you for absorbing blows, squeezing past people quickly, and running to safety.
Likewise, people who bite, gouge, scrape, poke, twist, and use weapons can inflict a lot of damage a lot faster and with less effort than one would need to procure a triangle choke or develop enough speed and technique to knock someone out with a punch or a kick.
Bottom line: street fighting sucks. Nice clean fights are far and few. Better to fight in the gym with someone you respect. If someone on the street loses their temper and wants to exchange sloppy blows with you and you don't think they're going to pull a weapon, poke you in the eye, or call over five of their friends, I suppose having some sort of mixed martial arts training would be helpful. But then the police show up and people take sides. Can't help you there.