It doesn't matter if something is illegal. People who want it are going to get it. The problem isn't people using a drug. Thats their business. the problem is the law enforcement and incarceration costs of investigating and punishing it. I'd be surprised if there was any meaningful increase in weed use now that its legal. A lot of the appeal of that sort of thing comes from its formerly illegal status. Once the novelty of it wears off, it'll be about as interesting a new variety of Swisher Sweet.
There is simply no logical reason for something like alcohol or tobacco, which cause literally hundreds of thousands of deaths each year, to be legal, and something like weed, which is relatively harmless, to be illegal.
Marijuana is not a dangerous drug. I'm pretty sure a cheeseburger habit is about 100 times worse for your health than smoking the occasional joint.
The internet is full of sources that will tell you that smoking the stuff turns you into some kind of monster or something. Its all agenda-based, and its all lies. Don't be a fool. Use common sense.