I think if it was legalised and the production of it was controlled so that there were no harmful substances in it, it would not be a problem to society. I smoked weed as a teen socially and then every day from being 21 until I was 30 and then I quit cold turkey. I never had a problem with it, I held down 2 jobs - one full time and one part time. I was always motivated and did what needed to be done and then some. I don't drink and I found weed was my release. In the past few months, I have been having some relationship issues and I have smoked the odd one here and there and I feel like it just helps me take the edge off the stress in my life.
I know not everyone has good experiences with it, but it's the same for every drug - legal or not.