Fenn, given the observable data from the Netherlands' policy of controlled substances, I highly doubt that legalizing weed will reduce crime rates. I never said reduce "drug use". I guess I should've been more clear: I meant reduce //drug crime rates//. Even if we see a decline in violent crime, which is unlikely, we still face the likelihood of seeing a dramatic increase in non-violent offenses. So, all in all, it's likely that //more// people would end up being arrested for petty offenses. So, even if we do manage to stop illegal selling and trading (unlikely) through legalization, it's projected that we'll see a massive increase in other drug offenses--multiple times our current rate, actually. I think we can agree that having more people in jail = bad, regardless of the crime.
And legalization itself is going to require massive reconstruction of our infrastructure, which will cost a lot of money in itself. Given that we follow the trend of the Netherlands and start seeing multiple times the number of drug arrests, the amount of tax payer money spent on maintaining penitentiaries and supporting prisoners will increase exponentially. Also, we'd have to pump more money into the FDA to research and regulate marijuana and put it on shelves in a manageable fashion. We'd have to buff up law enforcement to enforce a new set of tight regulations regarding weed specifically. Sadly, we can barely enforce drunk driving laws. I mean, look at how much money is spent on anti-drunk driving campaigns alone.
And, chances are, legalization won't stop illegal growing/trading. First of all, weed dealing is surprisingly lucrative to whoever can manage it. It can be assumed that the street value of weed will drop if it's widely available, but chances are people will still make a pretty penny off of it. Why? Because of the age demographics that smoke pot. The average age for pot smokers is 17 years old, which is still underage by Netherlands standards. Also, 15 is the median age at which people start smoking weed. It gets worse: this source cites that 8.8% of 12-year-olds smoke weed on a regular basis (at least once in 30 days). The 10-14 age demographic for the US is comprised of over 10 million kids. So at least half-a-million 12-year-old kids are smoking pot. So yeah, there is still going to be a lot of illegal trade given that a large portion of the weed smoking demographic is underage. Beyond that, we'd have to integrate licensing procedures so that we can license people to grow and sell the stuff (like in the Netherlands). We'd basically be setting up a whole new economy specifically for weed.
All in all, I'm pretty sure that more crime will result from legalization and much more money will be spent.