i think on the most basic level, to be recognized as a disorder, the condition must cause significant disruption of multiple areas of functioning (social, academic, occupational etc). in my experience, and from what i've read about others on this site, that disruption is the reason a lot of us ended up on this site to begin with. like other disorders, MDD can be co-morbid with other disorders, or occur due to the presence of another disorder, or it may exist on its own. i think there's def certain aspects of maladaptive daydreaming that would qualify it as a disorder -- although i don't think research has caught up yet, so that might not happen any time soon. 

I think it's more like alcohol or marijuana than like heroin. Most drugs are addictive for everyone who takes them. You can't use them moderately. But alcohol is not like that. Most people who use alcohol aren't addicted to it; they just like a drink sometimes. Alcoholics are a subset of the drinking population and we still don't know why some people progress from social drinking to addiction.

It's the same with marijuana. Most people who smoke it aren't badly affected, so it's generally considered a harmless drug, but I had a friend who got addicted to it and it made her psychotic.

So there are people who are addicts and whose lives are ruined by their addiction. But there is a wider tranche who just indulge in the drug because they can afford it and enjoy its effects, but aren't hooked by it. Maybe a hypervivid imagination is like that.

Browsing around this site, I've seen lots of posts by people whose lives have been completely ruined by MD and I feel desperately sorry for them, but that hasn't been my experience. I've always been able to give priority to the things I needed to do to survive. It was the rest of my time that I spent on my internal narratives, time that "normal" people would have been spending on social life or romantic engagements, or raising children, which were all things which didn't much interest me and still don't.

I see myself as the MD equivalent of a social drinker. But maybe I'm just bloody selfish...;-)

MD is very deep. But I don't think it's a disorder. I think it is more of an addiction because you aren't comfortable with being who you are. 
MD is everything that you couldn't process with your ego. It's sign of an underdeveloped ego. It's a sign that something shook you so hard in life that you chose to shut your self off and be in the head all the time. It's not a disorder I believe. It's like smoking addiction but it's covering something far deeper than just stress.