I think this topic was done to death in a thread awhile back but i don't think its an entirely good idea, yes it has its advantages but it has one big disadvantage (for me personally).
I think people who haven't used it or who wouldn't of other-wise used it would use it once it became legal. If it became legal it'd also be more socially acceptable for more people to do. It's sort of like a loop that re-enforces itself.
(I know its too late for alcohol prohibition, ect but im not using that argument) If Alcohol was only available on the black-market and not legally i probably wouldn't drink it (i do now, nothing against alcohol). By removing something off the black market and selling it you make it more socially acceptable and less of a stigma. This tends to make more people take part in it and clever marketing and social pressures have made drinking alcohol/ going to the pub with mates a "fun" thing to do and something acceptable.
Where as "I'm going to light up a few cones in the corner with a couple of my mates" doesn't have quiet the same acceptability as "We're going down to the pub for a couple drinks".
I mean sure people should be able to smoke weed if they want and if they get caught experience minor penalties but not a criminal record.
What i don't want though is us creating a bunch of government sponsored crack heads or the associated public health concerns with people who smoke marijuana. Suppose my proposition is right and more people take it up, it does cause harm in the long term vs not smoking it. These people who have been afflicted with conditions because of smoking it now have to be treated in public hospitals which we pay for with taxes, more than otherwise would of been the case.
You also have the problem of if more people smoke it, more people might drive high ect.
That said i'd rather people get high off their face rather than drunk, because drunk people can be very violent sometimes where as if you're high you just kind of mellow out (as far as i know).
I don't think full blown legalisation and/or having pharmacies dole it out is the way to go.
Decriminalisation is the best way. It keeps potential users more suppressed and keeps the negative stigma attached to it. At the same time, we aren't throwing scores of people in jail or ruining their names and future job prospects by giving them a criminal record for smoking the odd joint now and then.
My problem isn't with people actually smoking it, if they want to go straight ahead, my problem is more with having to spend additional money on this and creating more addicts than otherwise would of existed.
(Decriminilsation for those who don't know means we still keep it illegal but if you get caught, its not a criminal offense and you dont get a criminal record. You just get a minor fine or something like that.)