What is the argument for why weed is bad ?

This.

Stoners are the most chill people on the planet till they can't source weed. Lived in a house full of stoners at uni and everything was chill until we couldn't get a text back from our dealers. Then we were at each other throats.

That's not to say weed itself is bad as a one off experience (once or twice a year, tops) but it can very quickly devolve into frequent use if you get too suckered into the lifestyle.