the source is not very reliable(some obscure journal in portugese),
A very small percentage of people are smoking it for health effects. The rest just want to forget about whatever is going on in their lives and get high. Why can't people just admit the truth?
Many people consuming alcohol are doing so for similar reasons.
I'm just curious, your opposition seems largely centered on people being high and not the medical effects, are you a prohibitionist too? What about self-induced altered states of mind, hypnosis, etc? Neurochemically active foods (chocolate->dopamine link)? What's the reasoning?
Some Googling finds this: Pope HG, Gruber AJ, Hudson JI, Huestis MA, Yurgelun-Todd D. Neuropsychological performance in long-term cannabis users. Arch Gen Psychiatry 58(10):909–915, 2001.
Your caveat about correlation and self-medication could be right. It might also be wrong - you don't give a good citation either. My point was that I think we have a lot less knowledge about the effects of these drugs than e.g. alcohol which we're (ab-)using since millenia, so the UK analysis is certainly an improvement.
Citations, please.
A phrase to remember when citing anecdotes about mentally ill people taking Drug X is self-medication. This is a technical term in counseling psychology; it refers to a very common phenomenon where a person who is suffering a mental health issue -- which can be anything from a mild acute depression to full-blown schizophrenia -- figures out, by trial and error, that taking a certain drug seems to alleviate their symptoms, or make those symptoms more tolerable. This often causes the patient to end up with two health problems, which can be difficult to sort out from each other and which generally must be treated together.
So even if you uncover evidence that mental illness and taking drugs are correlated, you can't simply assume that the drugs cause the mental illness. It could just as easily be the other way around.