Hmm, drug addicts have an illness, and should be treated in a medical facility, in my opinion. You don't put sick people in prison, as this does not cure the problem, just compounds it.
In a country where alcohol abuse does far more damage to society (public order offences, criminal damage, marital abuse, etc.), do we stick drunks in prison? No, they get councelling. Drug addicts - the majority of whom actually keep their addiction private and do not go rampaging through your homes stealing you property, as the Dail Wail might have us believe - have to keep their problem to themselves, and often do not seek help for fear of recrimination from their families or friends.
And I reckon you'll be surprised to learn that most addicts (not kids you might ee on the strets who dabble with a 5 quid wrap of coke) are white-collar workers, who turn to drug abuse and subsequent addiction, as a result of their hectic and stressful working environment. Many do not take the addiction rehabilitation route as they do not want to replace one drug with another. And a 40 quid iPod is cheaper that 100 pounds worth of methadone, or other substitute, so if it works... great!