Like max said, I've read some supporting evidence for a kind of dose-regulation reflex that most people naturally do without thinking about. Basically, if someone is smoking lower THC weed, he will want to take more to get the desired effect, while if he's smoking hella strong stuff, then he'll usually wait longer between bongs/vapes/spliffs, so the net result is a fairly similar overall level of blood-THC levels between the two strengths of cannabis.
The main difference between weak and strong stuff is how high you immediately feel after the first toke, which can be a bit scary if it's way more intense than you expected.
The thing that bugs me the most about the 'dangers of cannabis' argument, though, is the way that the media and authorities present the idea that Cannabis can cause Psychosis and Scitzophrenia as a fact, wherea it is still very much a theory (the difference between correlation and causation), and if anything, the evidence shows that while cannabis can trigger psychotic episodes in people who have a high genetic predisposition to severe mental illness, it does not trigger it in people who have a low genetic predisposition (a category the vast majority of people fall into).
I once saw quite an interesting documentary where they talked to a professor who was investigating the matter for the UK government, and he simply said that in the last 30 years, cannabis use in the UK has grown at roughly three times the rate of population increase, yet people suffering from psychosis and other severe mental illness has stayed level. It seems hard to argue that Cannabis causes severe mental illness when you're faced with a piece of evidence as transparent as that.
People also seem to forget that Alcohol is also similarly capable of trigger psychotic episodes in at-risk groups, yet it is fully available to all members of the public over the age of 18, with no controls on how much can be purchase in one go.