It's a simple matter of Liberty I've never smoked a joint, but who am I to say that others shouldn't be allowed to? It isn't harmful to a lot of people, and those who it is harmful towards make a conscious decision to smoke it. We know smoking is also harmful (though whether more harmful than pot is a matter of opinion) but we still trust people to make an adult decision about the effects it has on their health. Why should we treat people who want to smoke Marijuana any differently?
I see other people linking Marijuana to violence, massacres etc as if it's the only possible cause of crime. What about alcohol, medical problems or abuse as a child? Do we treat ban alcohol in case it causes domestic abuse, or segregate everyone with a mental illness in case they flip out one day? So why should we consider banning Marijuana to "prevent crime"?
Humans are intelligent beings who can make the right decisions most of the time. And we shouldn't discriminate against the many Marijuana users who are never driven to commit a crime, or don't suffer from mental problems as a result just because of a small minority who do.