Feasibility [of heroin maintenance programs] is not desirability. Heroin maintenance has a contradiction at its heart. Having chosen to prohibit the drug, society then makes an exception for those who cause sufficient damage, to themselves and to society, as a consequence of their violation of the prohibition. Society's decision is setting the damage level that entitles a user to access. It can require that an addict cause a lot of damage to gain access, which is expensive (in terms of crime and health risks) and inhumane. However, if it sets the barrier low, then access to heroin becomes too easy, and the basic prohibition is substantially weakened. That contradiction alone does not make maintenance bad public policy, but it does raise a fundamental ethical concern.Obviously, this is not the only precedent when our society ultimately rewards an individual for a persistent violation of its laws. Take illegal immigration: migrants who are enterprising enough to overcome the obstacles (physical and otherwise) that the United States erects around its borders are often able to naturalize. Illegal immigration is a perfect example of an extra-legal status-quo: illegal aliens are tolerated because the U.S. is addicted to cheap labor.
Similarly, heroin maintenance programs involve a trade-off: despite general prohibition, some addicts who are "persistent in their addiction" are allowed access to heroin. In return, the society receives the aggregate benefits of lower health costs (no more diluted black-market heroin, no needle-sharing, not as much overdosing), lower crime-fighting costs (addict doesn't have to resort to crime to finance his habit at black market prices) and a possibility of social re-integration of an addict back into the community.
But wait! Despite the benefits of the program, the ethical dilemma is still there! But, only until one realizes that an exception to a rule does not always involve a compromise with ethics. This particular case involves a prohibition regime that is largely detrimental both to addicts and to the society at large. Any hole punched in this regime that moves it towards harm reduction and more sensible drug policy can be considered ethically suspect only on a purely logically-abstract level, insofar as it represents a contradiction with the existing policies.
Another, fairly straightforward way of removing the contradiction would be either shutting down heroin maintenance programs (not a correct choice, in my humble opinion) or legalizing heroin. Of course, the problem with that solution (in addition to the obvious political ones) is that nobody is really sure what's going to happen: some say that if we legalize, we just might end up with a much greater number of addicts. It is a valid argument - there is no firm basis on which one could confidently argue that a spike in addiction won't happen. On a theoretical level, one trick to avoiding mass heroin addiction in a legalization regime is a fine line between making access to heroin hard enough so that only the determined seekers of the drug would bother, but not making it so hard that it is easier or cheaper to obtain it in the black market. Drawing that fine line would not be easy; however, we did it with illegal immigration: the amalgam of border patrols, fences, regulations and penalties makes sure that we are not flooded with migrants; however the restrictions are not draconian enough to prevent our economy from getting its regular injection of cheap workforce.