^^These are all very valid points.
The point is, no one is suggesting that people should be able to buy their heroin at the grocery store along with a bottle of wine; however, there is a broad area between this and total prohibition. As Rachamin said, some long term addicts out there are going to do heroin regardless of the consequences. The consequences as they stand now in the current climate of prohibition are 100 times worse than they need to be. Virtually all of the drug-related problems that affect society as a whole stem from the draconian approach to dealing with drugs and drug addicts. The health problems from chronic use of street drugs, prison overcrowding, spread of disease, drug-related crime, etc., could all be alleviated by taking a realistic approach to treating addicts rather than a moralistic approach. Addicts are treated as criminals that have a moral defect that makes them behave the way that they do rather than a medical condition.
If an addict is unable to get clean after several attempts the only option right now is jail or death, which are inevitable. It does not have to be this way. If someone could get their heroin by prescription in a controlled fashion similar to how its described the rachamin's post, and that person is able to live a productive and normal life, that's one less problem for the system to deal with. It would also be cheaper for society than locking the person up and making them an even worse criminal than they started out. In addition to being much healthier for the addict, not that the prohibitionists care about that.
Obviously this is a much more complex debate than just "legalize it!", but the fact is that the total prohibition approach is not working. It just adds to the problems, but because of the "moral majority", alternatives to prohibition cannot even be discussed within main stream society. Cancer patients can't even partake in cannabis to alleviate their symptoms, we're obviously several years away from the decriminalization of harder drugs like heroin. We actually seem to be moving in the opposite direction as evidenced by the current climate of prescription narcotics such as Oxycontin that are becoming increasingly hard to obtain for legitimate patients.
Bottom line is that the we need to move away from the "War on Drugs" and instead declare a "War on Suffering". The drug war just adds to the suffering.