As I said before, we're not a good representation of the population on this forum.
Whilst true... I really don't see myself as in the minority of weed users who can toke responsibly.
I don't know a single 'stoner' as in the stereotype form who bums around life with no aim, scraping between benefits cheques whilst using them for green and munchies...
The ones I come across are like me... heck... I even met two more at my new(ish) job!
Every one I know (dozens), use it in such a way that it has no negative effect on themselves or those around them (apart, of course, from the inherent downside of smoking any substance)... usually keeping use to weekends / parties / etc.
A good friend just visited me for a 'stoner' weekend... he'd just polished off his final exams, for which he had not touched the stuff for months... it's so addictive! What did we do? We got a big bag of munchies from the local shop, a couple of take outs and watched a bunch of crappy films... it's nice to relax like that once in a while. I'm normally quite active at the weekend and rarely get the chance to just sit and chill like that. It's simple, reasonable recreational use with no impact on the outside world... I know I'm repeating myself... but why am I a criminal for this?!
Why should the minority who can't control themselves have such a profoundly negative effect on those who can?
I may be lucky in that I've only ever met the 'good' ones... but surely I would have at least encountered one of the bad ones... oh wait, they'll be at home on the couch, not causing anyone any bother other than a small drain on the economy, which is abused far more by people who've never touched the drug!
There will always be substance abusers... whether whatever the substance is, is legal or not.
Why not at least give them the opportunity to know the source, know the contents (yeah, even if it's only cut with sugar/flour), know the quality... this simple argument should be enough to make it legal.
... that would lead to more research on the subject of what actually constitutes abuse in the first place...