"To say that the person had problems only REVEALED by drug use isn't always corect, and is sometimes completely wrong"
Well, I don't think that we're talking absolutes here.
There is alot of truth in drugs showing the "true character" of a person. It is easy to put up a fake personality of understanding, caring, a "good " person when off of drugs. Drugs such as coke, E, psychadelics offer you a look at some other aspects of your friend's mind.
I used to have about 10 "close" friends in my life. Every one of them I've done E with before. During those "high times" with some of those friends I really saw that they were not who they pretended to be. Every single one of those friends I am no longer friends with. Not because of drug use, but because I became aware of their underlying motivations.
I now have 3 closer friends now. Friends that have proved their worth over time and with ordeals.
When people decide to open the Pandora's Box of illicit drug use, they go in with the knowledge that they will be testing their strengths and exposing their weaknesses. I think we all know people for whom that Box should not have been opened. Some personalities cannot handle any drugs.