The naltrexone also removes the reinforcement / euphoria associated with using opiates -or alcohol,- making it a two-for-one treatment while it's in the client's system. If each use becomes less rewarding relapse becomes less likely in the long run. One can still get drunk on naltrexone, and can still take enough opiate to become stupid and even overdose... But it won't be euphoric. 

At $1200/ injection plus ongoing support / counseling, it's still a lot less expensive than daily street opioids.