I wonder how many of these 'researchers' actually smoke marijuana on a regular basis. I definitely found that a prolonged use had the complete opposite effect of "reducing anxiety and depression". I understand that many professionals use marijuana, but how many marijuana users are able to maintain a professional career?
Whether we are aware it or not, everything that we perceive and consume effects us. One person will react differently to the sight of a bowl of cereal than another.
It is the same with drugs, but just more blatantly obvious. Even though there is no physical damage done, there is still a change occurring. Whether this change amounts to a damaged psyche or enlightenment is really hard to measure since the only precedent to hold our experiences against is anecdote.
Is there a reliable method to harness the positive effects of drugs? Probably, but it is a long way out there.
A person is indulging in a risk by taking drugs; but, there is an inherent perception of risk in most new experiences. Ever meet someone who is afraid of a new kind of food? Actually, I don't know if that's related, but whatever.
So, to answer your question. It is not always the case that drugs will effect a person negatively, or positively. There's a lot involved, but the laws simply do not allow us to understand the specifics of what's involved.