Well, if taxed right cannabis could help to fund the NHS, and to be honest (in the UK anyway) dope is so easy to get hold of, and so tolerated by many police forces, that outright legalization is unlikely to massively increase usage.
I doubt that there are that many people out there thinking "I'd love to have a spliff, if only it where legal".
I simply do not know. I know people who won't try it because it's illegal, and the vast majority of householders wouldn't have the first idea where or how to get hold of it. But you are right, it's possible outright legalisation won't massively increase usage. The difficulty is, you can't make it legal on the sly, so the publicity would in effect amount to an endorsement. And as the govt. spend so much combatting tobacco smoking, would it not be taken as politically hypocrital? I do wonder if that's a motivating factor and the status quo is the best compromise available.
The problem is not that marijuana shouldn't be illegal, it's that you'd have to make it legal when you could just tolerate it like the police in the UK do, as you point out. That way, those who want to smoke it, do, and those who don't because they can't get it, or because it's illegal, don't. Which works, I think.
Slingblade, I was posting that as a dim and distant memory of something I read which may or may not have been govt propoganda. I'll do my best to find a source for it but in the mean time just take it as my own opinion. I think it's highly probable that the legalisation of marijuana will lead to an increase in usage, but I couldn't say by what margin. But given the penetration of alcohol and tobacco consumption in the general population, I see no reason why marijuana consumption wouldn't eventually reach the same levels as tobacco (currently at 30% I think) or alcohol (no figure for that but I'd guesstimate at least 60%).
I will try and find some figures to back that up but I think any projection about increase in usage is pretty useless until someone actually tests it. And that won't happen. If the benefits of legalising marijuana outweighed the disadvantages, it would be done already I think.