In Mr. Black's article, he stops short of arguing that cannabis should be legalized, but gives a number of statistics that suggest that prohibiting it has been an expensive failure. The only downsides he points out fall into half a paragraph where he also dismisses the argument (using statistics) that cannabis use is a gateway drug. Even the negative effects he attributes to cannabis he characterizes as, "less damaging and dangerous to society than alcohol or tobacco."
Among the cannabis characteristics Mr. Black considers "less damaging" is "psychological problems." I find "less damaging" as strange a euphemism as referring to drug use as "recreational". Various medical reports link cannabis use to depression, impotence and other problems. In a time of worldwide financial trouble, not only are we left with the medical bills for these problems, but our society loses out on these individuals' capabilities to create, to better, and to participate.
To argue that cannabis is less damaging than alcohol or tobacco is to effectively make alcohol and tobacco the gateway drugs to cannabis. The former are allowed, goes the reasoning, therefore they should all be allowed; we're already there, and this isn't any worse. And down we go on the slippery slope.