Then to tie this in nicely which the argument that people with issues will ignore them - I went through court during school for restraining orders, I worked a full-time job while in school, and started a small profitable business. All of that is insanely stressful and I would argue if weed would have caused issues, I would have had larger problems.
All that being said, I think people who smoke weed (or tobacco for that matter) are more likely to get into the "drug culture" of partying, carnivals / festival, climbing, etc. It's really no different than people who become alcoholics. It's probably a symptom of a larger issue when people abuse drugs and attempt to "opt-out" of society. That being said, perhaps some people want this, and it should be an option. I don't care one bit if people want to waste their life on drugs. I only care that I have to pay for them via tax dollars, so remove social welfare for them and I'm all for it!
I'd even expand my argument, and say any vice including videogames, cars, etc. Anything can get out of hand if people are trying to avoid a problem. Just like you can't blame a gun for murder, you can't blame a vice for a wasted life. At some point, if a person had choice, and knew what they were getting into it's their fault.