It may work for grass, it sure as hell wouldn't be the same scenario with heroin/crack.

Crack, yes, induces tends to induce psychosises in a high proportion of its users that are dangerous to both its users and those around them.

Prior to its criminialisation,(under heavy US pressure) the UK had a few thousand herion addicts, a majority of whom were Doctors prescribing it to themselves.
It is not a good thing to be addicted to, but if it cost as much as a pack of cigarettes, people would not not forced to choose between their next dose, food or the rent. And not be risking prision.