If you're hoping to reduce use amongst the general population, legalization seems to be a better course of action than prohibition.

I don't think either of us are going to argue that no matter the legal status of marijuana there's always going to be a demand for it and a market. If you're primary objective is to mitigate harm to the public writ large, and not just scorning consumers, I can't think of anything more conducive than allowing for people to be open about their consumption habits without fear of legal reprisal so that healthy social norms and boundaries can be established.

Further, if that market is always going to exist I'd imagine you'd rather see the proceeds be cycled into legitimate enterprises that actually support society as a whole and contribute taxes rather than the black economy, which is where it's inevitably forced.

The pros far outweigh the cons here.