Who will the drug dealers sell it to?
And it is less risky. I'd support a system that gives I'd more junkies of the junkies aren't destroying their health, aren't stealing, have jobs.
But of you're only allowing heroin use for dependent users in a clinic, you won't destroy the black market. There are still people developing new heroin addictions in Switzerland. And not only new users will use the black market to get heroin. Anyone who wants to use it in their own home, anyone who wants to keep their use secret will have no other choice.
As another commenter mentioned, I'd imagine a lot of drug trade (maybe not in heroin, but definitely in cocaine) is not hardcore drug addicts, but occasional (or even regular) users who live (relatively) normal lives.