In my own anecdotal way I've worked with a lot of addicts who were 'cured' but still consider themselves addicts. They admit that there will always be temptation to lapse, even if it's 20 years down the road. Hell I've worked with people who have done that very thing, all it takes is something to throw their world askew and it's easy to fall back to. No it's not likely that you leave a drug addict alone with a big plate of cocaine they'll sniff it all up, but if you manage to do it at exactly the right (wrong) time in their life they may well do.
It's somewhat ridiculous to say that every 'former' addict is just a relapse waiting to happen, but it's also ridiculous to say that just because some years have past they'll never fall into it again. Hell even with the best treatment money can buy I've seen otherwise healthy looking people relapse back into it.
A woman I work with now, her sister-in-law lost everything due her alcoholism. Her family had to leave her, she lost her job and her nice home. But she put things back together for herself, went to rehab and sought out counselling. This was years ago, then just this month her Mother died and she went straight back to the booze. Now you can argue that her treatment was ineffective, personally I don't think it was. But with some people the need to drink, or smoke or fuck (Don't get me started on the short shrift that sex addicts get) is too great that therapy simply can't help them.
The problem when it comes to things like this is that blanket statements are all too dangerous. Just because A does something it doesn't mean that X, Y and Z will do it the same way, that's what can make a therapists life difficult.
An addict can always relapse, most don't of course with the proper treatment (Which is all about identifying problems and knowing that their crutch isn't the answer) but there's always the chance.
As for Dr Drew, I like him quite a bit. Just take a listen to the Loveline Archives online to see how good at his job he can be (It's really quite staggering just how quickly he can diagnose people, or spot a fake caller - though funnily enough through Osmosis Adam Carolla gets pretty good at it too, but even he seems wowed by Drew at times). Though the shitty reality shows he does kind of cheapens his work a bit, doesn't mean he shouldn't be listened to. His first book 'Cracked' is an interesting (And occasionally harrowing read). Andy Dick of all people was probably right when he said that Drew's problem is that he probably cares too much. He clearly sees Celebrity Rehab et al as a way of educating people, the problem is that you can't do that on a VH1 show because you're dealing with celebrities who have their own agenda, and he's pretty much admitted as such. If he had made the show with real people then it would've helped his case, but then it's less likely to get aired. Doesn't stop him doing it though.