I don't agree entirely with your What would happen if it were legal and what would happen if it were illegal I have to say.
In my opinion, I think it should be legalised. I don't think it would help to stop underage people getting their hands on it, when have you ever seen a 14 year old kid struggle to get some cigarettes? That said though, I think it would have a few benefits that outweigh the downsides.
For started, it would take the criminal element out of it, this is always a good thing. It also means that it can be taxed, increasing government income.
I also do not think it would lead to many more people smoking than already, I know of a large number of people who smoke on a regular basis and they never have trouble getting hold of some should they wish. Therefore, I would imagine that even if it were legal that a large portion of people who do not smoke it would keep that way.
There are a few aspects I'm not sure of though. For example, is it possible to test for somebody to see if they are under the influence of weed straight away? It would be important for that to exist in order for it to be legalised in order to enforce laws against smoking and driving for example. There's also the possiblity that due to taxation it could end up costing a lot more than it currently does from the black market
