PSH death.

Argument I have a hard time understanding in the wake of PSH death: 

Heroin is somehow used because it is considered "cool". 

If we discount the (common) prospect of people who become addicted to opiate painkillers who moved up the ladder to heroin, the overwhelming percentage of any substance abuse is out of self-management of sometimes very serious mental disorders and the associated social problems. Since heroin has an addiction potential that's roughly that of nicotine (eg, very high relative to many other drugs), and since it is illegal and thus difficult to get in a pure and reasonably safe unadulterated form, it's really hard to imagine people rationally deciding one day "yes that sounds good, let's try that" as a large base of its use. 

Not to mention that there are almost zero positive cultural references to it. 
Consider the following movies and TV shows. 
Pulp Fiction - famous OD sequence. 
Trainspotting - famous for dead baby. Famous for toilet diving sequence
Requiem for a Dream - famous for sex sequence involving public sex and a double dildo
The Wire - Various homeless or near homeless characters are the only users depicted, dealers/bystanders being shot or shot at, several overdoses, hot shot with poison involved
American Gangster - gang war, police corruption, main character selling goes to prison
Traffic - police corruption, daughter becomes addict, steals jewelry, trades sex for drugs

Ray might be the closest heroin comes to getting a "this is okay", and even there, someone ODs and Ray is using for a basis other than creative potential in musical style and has to clean up his addiction both for legal and personal reasons. 

Compare this to cocaine, which may be used by sketchier characters certainly but seems to have very few negative consequences of these types (both in movies and to some extent, in reality). Sherlock Holmes used cocaine. Jordan Belfort uses cocaine and a bunch of other stuff. Raoul Duke does cocaine and most of pharmacy. Malcolm X does cocaine. Scarface has a huge pile of the stuff. And so on down the list. Sometimes this has negative effects to the plot, sometimes it just establishes the character as a bit edgy or strange. Rarely is it "someone OD'd on cocaine" or "someone does something totally absurd", as is common with a marijuana or alcohol reference. 

Hoffman like many artists in a now stereotypical description sounds like he was somehow troubled personally in order to create and hone his craft. There's some correlation with artists, which skilled non-Tom Cruise actors are, and various forms of depression, mania, anxiety, and so on through the litany of mental woes and abuses that we may inflict upon ourselves with our minds. I imagine for actors or actresses it is particularly likely as the skill set for concealing depression from our friends or loved ones while in public is essentially the same as acting. Better practiced perhaps, but similar. This doesn't mean that people in Hollywood use drugs or have drug problems because it's cool and awesome. It suggests that people in Hollywood use drugs or have drug problems for much the same reasons people elsewhere do but that the concentration of creative people means a couple problems emerge.

1) It's more readily available because some of these people are earning a lot of money and could "afford" a drug habit
2) It's more readily available because there are more people with such problems in one place and who co-mingle to work together. 
3) It's less likely to be seen as an impediment to the work than the underlying conditions that they're using it for. 

That last is possibly true for people in other professions as well, those are just more often professions that will drug test employees and fire them over negative test results. If we understand drug abuse, and addiction as a portion or a symptom of larger mental health issues, it's harder to stigmatize the depression or bipolar disorder or any of various social anxieties and perhaps easier to send people off for healthier ways to manage these concerns rather than they remain unacknowledged.