The negative effects of alcohol aren't evenly distributed, some people are more susceptible to alcoholism than others.

I would expect weed to be the same. As such, I don't just want to know the net negative effect, I also want to know the distribution. AFAIK weed can agitate certain mental conditions - the average joe can enjoy weed, just the expense of a minority losing their mind. I might not find that acceptable.

There's also an argument that society should seek to decrease chemical over-indulgence in general, including alcohol, and maybe fat/sugar too; legalising another drug just increases the battlefront perimeter.