The only benefit of prohibition is to the gangsters. With all the obvious side effects, contamination to increase weight and so profit, increased crime etcetera. Law abiding citizens are forced to associate with criminals thus increasing the likelyhood of thier being introduced to more dangerous substances or becoming a victim of crime themselves. It is an unnecessary burden on the police whose time and resources could be much better spent elsewhere. Having to enforce an unjustifiable law also makes them unpopular, leading people to not co-operate with the police and even to protecting criminals. This law makes criminals of millions of innocent people with absolutely no benefit to our society as a whole. There is much to be gained by repealing this law and nothing to be lost.
Prohibition does not work, it never has and it never will. Despite the millions of pounds spent trying to prove otherwise cannabis has no dangerous effects. It is true to say that some people who have tried it have found it unpleasant and some have an adverse reaction, but these are a tiny minority. Millions of otherwise ordinary productive and successful people regularly enjoy it. It also has many beneficial effects- especially as a pain reliever and is a sensible alternative to alcohol.