Methadone is pharmaceutical heroin. Switching addicts from heroin to methadone is a way to have their addiction controlled so they are not struggling for that next fix on the street. It trades one addiction for another. Methadone is actually stronger than heroin, it is very long acting and the metabolites stored in the bones. That is why detoxing from methadone is worse than heroin, their bones feel like they are breaking...
I thought the idea behind methadone for heroin users - was that it blocks the "high" that heroin normally gives, so the users have less cravings to seek it.