Go and visit your local mental health hospital and take a bit of time to study the people who have had their brains fried by smoking the shit, then you might see the justification for keeping it illegal.
Many of them will be well below the age of 18 by the way.
This is a subject very close to my heart, and I find it frustrating when people are glib about it. 
It is not a case of being glib about this subject but the fact is the war against the use of cannibis, with cannibis farms apparently in every street, is being lost at great cost. Why not legalise it and control it's supply, whilst raising £6 billion of revenue to help improve both the NHS and Mental Heath services. This could reduce the cost of the stuff, making it unviable for criminals to produce and distribute it, let alone controlling the quality of cannibis which is now being produced at far higher strengths than it was. It would also help those that need it for medical reasons.
The truth is that police resources are being wasted by chasing what is a never ending problem, that cannot be overcome without a new approach. Legalisation would also take away any perceived "glamour" and attractiveness of a presently criminal, against the authorities, act that many seem to have (I have myself dealt with many cannibis users and know what goes on in the heads of the young especially). It is a habit which many of the older cannibis users, like acoholics and smokers, would love to lose, so extra funds to the NHS could help their aims without the costs of police time, court services, and prisons. Currently many cannibis users are on on a merry go round of smoking the stuff, being warned / arrested by the police, fined, then back home to smoke it again, with further police action, etc, etc. The police see this constantly so this cycle must be broken by a new approach to help all.

Let's also recognise that the use of tobacco has fallen dramatically by using a controlled, governmental process since the 1960's, so why not try the same with cannibis.