I've seen too many horrible things happen to people I've known. Please don't tell me that smoking cannabis doesn't lead on to other drugs. It can.
I don't use drugs apart from alcohol and believe that cannabis should be legalized. It's already illegal to drive unduly impaired. Legalizing and instituting mandatory piss testing for employment is moronic; btw, it's not all jobs that are piss tested, and the ones that are are mostly those done by the poor. The war on drugs has a class dimension too.
I also believe that for some cannabis use does lead onto the use of other drugs but would also say that that is: a) a link on the chain of mood-altering substances that comes chronologically into people's lives after sugar, alcohol and caffeine, and b) partly true because the very act of buying cannabis brings you into contact with drug dealers who may or may not also have other drugs for sale.
The way to minimize the harm from cannabis is to take its distribution out of the hands of organized crime business groups and states, and put it into the hands of a highly regulated, highly taxed transparent market. The criminalization of drugs doesn't just create gangsters in the UK (or other consumer markets), it destabilizes and corrupts entire developing countries and costs lives from underdevelopment.