Amazes me how many people think that legalising it would change anyone who tried weed into a slavering useless addict only capable of thinking about the next joint and the next snickers bar.
Sure some people lose control, and can't moderate their intake, just as in alcohol, or sugar.
I think we need to put legal and illegal aside for a moment. I have no stats to back this up, but I'd suggest that, just as most drinkers have a few drinks a couple of times a night, enjoy getting slightly buzzed and go about their life, most weed smokers have a couple of joints every now and then, relax and enjoy life.
My best friend used to work in insurance, and it was an eye opener to me how many well paid, high responsibility 30-50ish workers, including high management (pun not intended! ), used to dabble in the weed, and yet manage to go about their day and still operate.
Again, it's not as dangerous as alcohol or tobacco, and doesn't endanger others like they do. The reason it's illegal, is 'the public' think it's in some way less 'moral' than the legal highs, and that it would destroy the alcohol and tobacco industries as an alternative.
The tax thing is difficult, as it's fairly easy to grow yourself, unlike tobacco and alcohol.
I imagine they could make it cheap enough to make a stack of cash and yet not worth the hassle of maintaining an indoor garden however, after all, most of us won't brew our own beer or have a setup to distill spirits.
Oh and we do have paid assassins, although I don't know for sure which way they affect the economy, but the army is that. I'm not knocking the army here btw, they're generally honourable people who go into the career for the right reasons. However they're paid to kill people we don't like, in short.