Yes but are they being widely used? To what extend will its usage increase? What impact will the increase in users have on society as a whole? How will society manage to keep on functioning and keep the economy going if a substantial proportion of it is limited because of the drug? What are the chances of the legalisation of this drug giving rise to massive social problems like those present in Iran and Turkmenistan? Does the society have enough resources to tackle any similar disaster?
These are the questions that should be tackled.
Yes they should be tackled in the context of
how we end the Drug War and reconstruct drug policy not
if we should do it at all. "It's not that simple" is the common refrain of those opposing immediate reform to broken/unjust/corrupt/inefficient policies and institutions, but, in terms of a commitment to ending the Drug War and constructing government drug policy based on reason and justice rather than fear, narrow institutional interests, and moral panic, it really is that simple.
That commitment should come first and foremost, and should be clear and unwavering. At that point it is certainly wise to discuss the potential negative consequences of legalization/decriminalization of certain drugs, and the best ways to mitigate these consequences-- I've thought about this often too, and have constructed a variety of regulatory scenarios in my mind that could mitigate such consequences (mostly based on government rationing schemes-- which I understand wil lead to gray and black markets, but ones that will be much more controllable than those that exist now).
However, that discussion of the negative impact and how to limit it, should not be a prerequisite to making a commitment to end fundamentally unjust and destructive laws, policies, and institutions. The same consequentialist/incrementalist/"it's not that simple, let's examine this more closely first" objections have been used in response to such things as abolition of slavery, universal suffrage, desegregation, and end to discrimination against gays in the military, and just about every other meaningful effort at repealing unjust laws and reforming broken or unjust institutions/policies.