The most effective argument is that it is a "Gateway Drug". There is no proof that weed makes people use crack or heroin but it is a good way to scare the public.
Legal marijuana makes it so that people don't start using more dangerous drugs on a whim. They are no longer presented with an easy opportunity.
If pot is on the same level as alcohol which do you think is more of a gateway drug? One that makes you introverted and cautious or one that makes you more confident while simultaneously impairing your judgement?