Many students of traditional martial arts are surprised to learn why martial arts aren't good self defense. Though there are many benefits to martial arts training and self defense training can be augmented into a martial arts training regimen the two are exclusively different. Below are some of the reasons why martial arts aren't good self defense.
Reason one, limited space. Close quarters fighting often negates the advantages of martial arts that can be utilized when fighting on an open mat. In cramped spaces the fighting style has to change by necessity. In a brawl virtually any aspect of the environment can be used to cause injury be it table tops, chairs, bottles, doors, or pavement.
Reason two, no referee. In a melee, there is no referee to penalize cheap shots or two enforce the respect between two equal opponents. Granted many brawls will be broken up in a relatively short period of time if bouncers or the police are called in there is no win by points and definitely no tap outs.
Reason three, like any style of fighting martial arts has inherent weaknesses. Bruce Lee developed a new fighting style designed to reduce what he perceived to be the main weakness of martial arts, which was that form was the enemy of flow. The highly artistic and athletic postures and stances of martial arts can be exploited in a close combat brawl with no rules and make shift weapons involved.
Martial arts classes can increase your physical fitness, make you more comfortable in kicking or punching your fellow man, and conditioning your body to absorb a blow with less injury. Though there are several reasons why martial arts aren't good self defense there is no reason not to pursue martial arts training if you want to because of ways in which martial arts training can add to the benefits of self defense classes. However, keep in mind that for real self defense, you need to learn skills that can be used in a real world situation.