You can make an argument for legalising it when you do a comparison against drugs that are already legal -alcohol and cigarettes. Both drugs contribute to societal harm, and can do significant damage to the health of individuals.
Were either drug to be introduced to the market today, it is unlikely that either would get a licence.
Some care needs to be taken though before we just legalise it out of hand. There needs to be an established method to measure blood levels, as with alcohol, and laws surrounding the use of vehicles and equipment whilst under the effects of THC.
And it comes with its own set of health warnings.There is some evidence to suggest that it does have some addictive/ compulsive qualities, and some evidence to suggest that you can build up a tolerance to its effects, leading to increased use.
If you smoke it, you run the risk of all smokers of cardiopulmonary disease or cancer. Prolonged and heavy use has been associated with an increased risk of psychotic episodes - again not something to be lightly dismissed. There is also research that suggests a deleterious relationship between neural development and IQ in teenagers - although it is unclear from the results whether it is a causative effect on the brain, or whether it encourages lifestyle choices incompatible with study, homework etc.
Other than that though - smoke away! :) Subject it to the same caveats as alcohol - Over 18, big fines/jail time if you are caught under the influence whilst driving or operating equipment etc.
The evidence would also suggest that it has a place in the treatment of certain types of chronic pain. .
http://www.bbc.co.uk/news/health-1937245
http://www.rcpsych.ac.uk/expertadvice/problems/alcoholanddrugs/cannabis.aspx
Prof. David Nutt, ex govt drugs advisor, thinks the net effects of legalising cannabis on society would be beneficial, although he acknowledges;
""Of course cannabis is harmful, all drugs are harmful. You can't have a harm-free drug, of course cannabis is harmful, but you have to be proportionate.
"The harms of cannabis are less than the harms of alcohol. Cannabis is not safe but I am saying in proportionate terms that kind of regulation would have a net population benefit on health."
http://www.independent.co.uk/news/uk/politics/exgovernment-drugs-adviser-professor-david-nutt-repeats-cannabis-warning-7866251.html