As an ex addict to (too many) substances (not marijuana) I can easily see a few cons regarding drug usage but the real pro, if I had to pick one, would be to remove all that money from drug dealers.
I would rather my money went to drug dealers than to capitalists exploiting addiction. I reckon we only finally got legalization because of government corruption and kickbacks. The idea of fucking Fantino (you probably don't know him, non-Ontario people) making bank after a career of using his power as a police chief to fan social stigmas and demonize users in the press all those decades is more than I can stand.