For met ex, methadone brings a level of normality to his life. He can go to the chemist once a week to collect it and then just take it every morning when he gets up, and it allows him to get on with his day. It eases the level of crazy. When he's using his entire life is drug seeking, he's incapable of being a 'functioning' addict. So although he's chosen not to use any recovery resources at this time, his current abstinence means he gets a break from the crazy train. So maybe it's a crutch, but it's a useful one.