Drug abuse and self-destructive behavior has been associated with the entertainment industry forever. It is especially associated with child stars as they mature out of one spotlight and enter another. The horror stories are countless: Tatum O’Neal, Drew Barrymore, Corey Haim, and on. Some are able to weather the storm, and others not so much.
I agree that for some transparency is good way to reaffirm clout. You mentioned Demi Lovato. She seems savvy enough to realize no one is going to buy any nonsense from her considering her youth and the scrutiny around her. Her honesty may actually play a role in keeping her sober. She may have learned from the walking traffic accident Lindsay Lohan that if you want to talk the talk, you better walk the walk. Miss Lohan has tried to downplay her destructive habits, and unreliability by lying through her teeth. It has only served to further damage a career that may never recover.
I do not agree with your assertion that Phillip Seymour Hoffman’s silence was apart of his overdose. Hoffman was actor first and foremost and avoided the celebrity spotlight as much as any high profile can. If he made an appearance, it was for his film, and nothing more. Unlike Miss Lovato, he was not a child star. He also had children and a wife, and possibly talking about his issues could shine a very intense unwanted light on them. As with all image maintenance, it must all be custom fit to the person in question.