I personally believe that prisons should primarily be used to house violent offenders and people who present a danger to law-abiding citizens. Drug offenders who use weapons as part of their makeup are dangerous and should be separated from the general populace. Large scale dealers also present a danger to the world around us, and incarceration seems a fit punishment. However, personal drug users--especially the occasional, recreational user who may hold down a job and contribute positively to his family and community--should not be thrown into the terrible prison atmosphere inhabited by those who should be alienated from society. If drug users do not pose a threat to those around them, why should they in turn face the dangerous threat of surviving prison life?
Personally, I don't think it serves the needs of the prisoner or of society at large to place drug offenders in prison. If your purpose in sending offenders to prison is to reform them, then the last thing you will want to do is to send a drug addict into a prison. This is for the following reasons:
1) It is known that drugs may be available in prison as they are on the streets.
2) A drug addict will not be able to access the help that they need to defeat their addiction and overcome it while in prison.
3) If you are trying to go cold turkey or overcome your addiction, the last think you need is to be surrounded by other hardend criminals who might lead you into more trouble.
4) Whilst it would get drug offenders off the streets, prison is not an effective long-term cure. It only solves the problem temporarily, whilst potentially storing up many other problems in the future for society.
This is, of course, a matter of opinion. My opinion is that putting drug offenders (so long as they are not violent offenders or big time sellers) is not a good use of prison space.
Drug offenders are probably best rehabilitated through drug treatment programs. Those who have been selling might be best helped by job training programs or more education. Either of these would surely be cheaper than the cost of incarcerating such people.
However, this is not likely to ever happen unless the current budget crises in states force their hands. Paying to help drug users or small-time sellers is not popular. People want to punish them and the programs to help them seem more like rewards than punishments.