Bumbaclot, bugger off. You have another thread to go and play on: this one is not about debating the ins and outs of cannabis use; it's for someone asking for support with a teen whose use IS a problem.
And yes, for the record, I have smoked it - regularly though my late teens and early 20s - and still do occasionally if someone hands me a spliff at a party. And no, it hasn't ruined my life.
But my 17yo son is different: he has done countless things he wouldn't have done if he hadn't been smoking so much skunk: he has dropped out of college; stolen over £1000 from me; got stoned enough to enter an ex-friend's house at night and steal her phone and be arrested for burglary; played with fire in his school building and risked burning it down; smashed up bits of my house when he's lost his temper; assaulted me; taken M-CAT several times because (he says) "it's a deng drug mum, I hate it, but when I'm stoned it just seems like a good idea"; climbed into a too-small cupboard 'to hide' and barricaded himself in our house and done other weird and irrational things; invited a convicted armed robber and drug dealer into my house when I was at work; sold all his stuff... I could go on...
I don't give a flying fuck whether the egg came before the chicken, and doubtless he has 'issues', but don't a lot of teenagers? And how can you tell if your own teen is going to be one of those affected until it's too late? Cannabis is a dis-inhibitor and makes him behave like an arse if he smokes too much; and it is a sedative and makes him unable to get his act together and get up in the morning; and imo skunk also makes him aggressive.
Like you, he doesn't think his cannabis use is a problem. 
Most of the adults he comes into contact with have no idea. They can probably smell it on him a bit, from time-to-time, but he's polite and even charming if when he's out and about during daylight hours... And most adults don't ever enter the 'twilight zone' occupied by disengaged youth :(
Bargainmad, I find it hard to 'walk the line' with money. Like maryz, my experience is that he steals if he hasn't got any; like you, I find it hard to bear the thought of him spending my hard-earned money on drugs. In the end, the 'compromise' I have found is that I no longer give him any 'money for nothing' - he has to earn it. He's lazy, so that doesn't happen very often... But I want to encourage him to work and generally 'make an effort', and I'd prefer him not to sit around all day every day, so on balance it seems worthwhile. In any case, he has a casual job, so can get money for himself sometimes, so it is simply beyond my power to keep him totally skint.
I don't think I'd tell the other parents, unless they're your friends. My friend did, and the other parents went into denial, and basically scape-goated her son, and his reputation was destroyed as they told everyone in the neighbourhood he was a dealer who had given drugs to their children :(