The binary "functional" vs "non-functional" is not a good way to understand such a complex thing. I don't care where on the spectrum you lie, if you're drinking substantial amounts of alcohol daily (this isn't necessarily the definition of alcoholic) then your normal activities and well-being are getting seriously affected.

You could use a similar comparison for suffering in general (chronic illness, chronic pain, etc.) Some may appear more functional than others while in such conditions. Just because you're functional with chronic pain, doesn't mean you aren't hurting and that your life would be very different without that suffering.

Edit: Spelling

Personal take: I think medicine has historically been about categorizing symptoms, patterns etc into diseases (adhd for example). I'm not condoning or encouraging or saying "go do heroin if it works for you" etc but... a problem for you may not be a problem for me - and its all relative to the individual. I've lived in Boston and Wisconsin and I couldn't stop laughing when I read how many drinks/week are considered to be alcoholic considering how ritual it is for a lot of folks in those areas.