Here is just the obvious. I know plenty of guys that do not train in any Martial system. They almost never get into any violent situations either. Doesn't mean they won't, but most of the time we are not whipping the guy next to us. One interesting stat claimed that most people would be in a position where their life would be threatened with physicl force at least once in their life time.
I teach and I train for many reasons. One of the most important reasons is that training in Aikido, Jiu-jitsu, and Judo is simply a lot of fun!!! You meet people you would not meet in other sports or activities. One of my friends, who is an expert at Brazilian Jiu-jitsu, swears that something has to be mentally wrong with a person who wishes to train in Jiu-jitsu or Judo (because those systems can be so very un-gentle). I often joke about what that guy was smoking when he came up with the name JUdo. Perhaps, he had a talent for sarcasm. No, Judo can be gentle. For example, after you've been choked out, you usually have nice dreams. I am going to get in trouble, so I am going to be quiet now.