Firstly, I should be clear that I disagree with prohibition of any kind. However, lets also be clear that cannabis is not without its dangers:
1. Smoking it is every bit as carcinogenic as tobacco.
2. For some people, long term use can lead to dependency and mental problems.
Many people I know - I'd even go so far as saying most - smoked during our younger years. This includes all kinds of professionals including doctors and lawyers. For most it was harmless fun and most have grown out of it. In at least one case I know though, it led to a failed marriage, lost job and time in a metal health institution.