"miracle cure for everything"
That's my main argument to why it can't be good for them, it's evidence how much the use makes them want to believe.
But on a more cynical note, I get the impression that it might even help fight global warming: just consider how small the ecological footprint of a stereotype stoner failure lifestyle would be compared to a median middle class career. Find a solution to the packaging of spontaneous midnight snacks (sorry for reveling in stereotypes) and it would be about as green as it gets, except suicide. Could we capitalize (well, "ecologize") on that?
(even more off topic ramblings from here on)
You could set up "fun monasteries", were people could self-admit to dedicate their lives to nonproductive but low-impact activities such as stonerism, abdicating from consumerism, career and procreation in exchange for minima food, shelter and the means to follow their low-impact hobby. Those "fun monasteries" could be set up not just for doing drugs but for all kinds of low-impact but sufficiently addictive activities. Video games, certain sports, artisanship, coding, even for nonreproductive sex, science, or, if you want to go full retro, for religion. I'm sure that they could all be cheaper and lower impact per head than excusing models of welfare that just treat everybody in need as temporarily embarrassed middle class. "fun monasteries" would not replace welfare, they would be an individual alternative to it. Just like the prison system already kind of is. Would there be a mass rush? I doubt it. Butt supporting people who might be willing to severely lessen their footprint in exchange for a nonmonetary stipend sounds like a no-brainer low hanging fruit.
(for those who read till the end and who were wondering if that isn't just a green spin on the "concents" in Stephenson's Anathem, sure, the idea grew when someone mentioned the concents in a different thread on the site. Before that it was just general disappointment in how much we still resent low-impact lifestyle choices, even in circles who outwardly agree on strong measures to fight climate change)