People have been using substances since before the dawn of civilization for various uses, including the induction of hallucinations, which were and still are, in some cultures, the foundation for religious life, for seeking either internal truths or external truths.
People have also discovered, through trial and error, the various uses for plants that could be used to heal or kill. Marijuana, though not a powerful hallucinogen, can be used to distort the senses and help produce a meditative and calming state.
People also use drugs and alcohol as a means to escape from the pressures of life, the misery of day to day existence in meaningless, unfulfilling jobs, and unhealthy relationships, or simply as a way to reduce anxiety in social situations.
The use of substances is ingrained in the human psyche. In today's world, some people are addicted to power, some are addicted to freedom from those in power. By keeping substances like pot illegal, it further denies the freedom to pursue happiness that we the American people have fought for, and further divides those who are in power from those who are not.
I think it's ridiculous to deny a person the right to ingest into their own bodies whatever they want, so long as they are willing to deal with the consequenes of doing so, are aware of the risks they take and do not jeopardize others in doing so.
What is freedom for if not being able to make your own choices about your own self?