Well, I think that a big part of those "functional" users are like functional alcoholics. They continue for many years to hold a job, but their daily usage causes problems for everyone around them.

At least that's the story you'll often hear at family group meetings. Not the usual homeless drug addict story, but people keeping up appearances while hiding some pretty big problems.

My brother was a functional meth user for many year before he lost his fiancée (mostly because of the anger outbreaks and lies, we didn't know he was using then), then his license and car, and finally his job. Of course then again there may be far more people that just continue using without ending up in the same situation.