Not everyone who smokes weed does harder drugs, but everyone who does harder drugs did smoke weed at some point. This argument makes sense and is not some version of "all spree killers eat bread, so bread must be bad", because the percentage of the population that smokes weed is relatively low (in Germany).
I am for legalization but I do allow myself to think differentiated, even about things I support.
I have seen people with psychoses caused by marijuana and I know smokers, who struggle to get something going in their lives.
I am not saying that weed is evil, I just know, that for some people it can be really dangerous.