Unless someone can present a credible case that increased Marijuana use results in increased crime, DUIs, illnesses, medical/insurance costs, etc. etc. etc. or that it is proven to be a "gateway" drug to the hard stuff. So far, I haven't seen one yet but I think more study is in order. We'll see what happens in the states where one or both are, presently, legal.
There are some that argue that legalizing (and taxing) recreational Marijuana might actually reduce some crime (i.e. the illegal trafficking, sale, etc. of it and the results of all that activity). That might be balanced by some increases in petty theft as people try and "creatively finance" their purchases of it, or attempt to grow their own (assuming it would still be illegal to do that), or deal with the backlash that comes from the "illegal" trade trying to fight to keep their business. This, as well as any net increase in Motor Vehicle issues (DUI), but, hopefully, no worse than those we have presently with Alcohol. I think it would be hypocritical for society to keep weed illegal for those "safety" reasons when, in turn, it tolerates Alcohol and all it's problems.
Personally, AFAIC, the jury is still out on both Medical and Recreational Pot. I would welcome any therapeutic value it might have in re: the treatments for seizures, glaucoma, oncological issues, etc. etc, but there needs to be a bit more history of success in those regards (and lack of problems). I'm also not yet convinced in re: Recreational weed, either. There needs to be much more study and history that it *doesn't* result in increased "problems...." certainly no net gain over the present level with Alcohol/Tobacco. Having been an EMT in a previous life (and a PIT) and having to bring several junkies out of some rather expensive highs (and taking a lot of $h!t for it - "HEY I PAID $$$ FOR THAT HIGH!!!"), it will take quite a bit of convincing. I probably won't partake, personally, if it is legalized (I never smoked Tobacco and haven't had any alcohol that I recall in more than 5 years), unless my doctor were able to convince me that it would be the one and only treatment available for whatever was ailing me a the time.
That's a lot of convincing.