At least as important are considerations such as risks to health, costs in terms of lifestyle, the morality of the act. But think about it. Suppose heroin were legalized tomorrow and available everywhere. Because you would decide that the pleasure does not outweigh the risks. As for current addicts, if they did use a little more, that would not necessarily increase their inability to function, or further impair society's efforts to deal with them.
Addicts already cause harm to themselves and present a danger to others.
But there is a limit to the amount of harm they can inflict on themselves and to the degree of risk they can present to society. As I said, controlled users believe their drug use would be unaffected by legalization. Because drugs are already widely available, controlled users could use more right now if they wanted to. They choose not to use more because they have determined how much they can take and still function. In other words, they regulate their own behavior.