Marijuana like any other drug or substance that is mind altering, is highly addictive to someone who has the addiction genes. Genes you say? Yes genes, do the research if you still don't buy into addiction\alcoholism being a literal disease.
After getting that out however, I don't believe marijuana being illegal will stop a marijuana addict from getting it. While it certainly would be easier for an addict to get their hands on it if it were legalized, if they aren't currently in a program of recovery (AA\MA\NA) anyway, then they are still progressing in their disease.
Same way that alcohol works, if alcohol was made illegal, the alcoholics would have no trouble getting a hold of it.
Just about the only issue I have with legalization, is, outsourcing. The amount of money made from the drug trade with Mexico and South America is ridiculous. Legalize in this country, and it's going to explode. So I would assume bringing weed into this country would still be illegal, however, if it's legal to use here, there's going to be an awful lot of people willing to do what it takes to get it here. We have trouble securing our border as is.