I support legalisng most drugs, including cannabis, but I do accept that sometimes those in favour can be too complacent about the negative consequences. There are strong arguments against legalising or decriminalising drugs, see James Q Wilson's article here for instance:
The central problem with legalizing drugs is that it will increase drug consumption under almost any reasonable guess as to what the legalization (or more modestly, the decriminalization) regime would look like. The debate, I think, must be between those who admit this increase and then explain why they would find it tolerable and those who admit the increase and find it intolerable.However any argument either for or against has to be rooted in the facts not in spin and propaganda. Ignoring reality is not a basis for making good policy.
Now what happens? Here is where the only meaningful debate can exist. Do you think that there will be a decrease in drug crime? Maybe—if the crime committed by users seeking money to buy drugs and the dealers protecting their right to sell drugs falls by an amount greater than the increase in crime committed by addicted users who are no longer capable of holding a job. Not all coke or heroin addicts are incapacitated, but a significant fraction—perhaps one-fifth, perhaps more—are.
This also applies to other controversial subjects. This week it emerged that the government had suppressed discussion of the relationship between crime and immigration when being honest about it would have avoided the dual dangers of complacency and hysteria.
The fact that fear mongers like Liam Donaldson remain in post and David Nutt is sacked speaks volumes.