Is martial arts good for street fights?

i keep hearing over and over again, it depends on the martial artist.. look if someone taked a few years of martial artist and earn those coloured belts they should be ready for one on one encounters right>? i just saw some kid who took karate get brutaly koed by some wild swinger... i understand some MA are for weapons or more than one opponaint, but im only talking to the one on one fights...