Anarchist essayist and all-around rabble-rouser Hakim Bey (aka Peter Lamborn Wilson) makes a convincing argument against legalization (and subsequent commoditization) of the mind altering substances our politicos have deemed A Bad Thing™:

For example, consider what might result from the legalization of “medical marijuana”—clearly the will of the people in at least six states. The herb would instantly fall under drastic new regulations from “Above” (the AMA, the courts, insurance companies, etc.). Monsanto would probably acquire the DNA patents and “intellectual ownership” of the plant’s genetic structure. Laws would probably be tightened against illegal marijuana for “recreational uses.” Smokers would be defined (by law) as “sick.” As a commodity, Cannabis would soon be denatured like other legal psychotropics such as coffee, tobacco, or chocolate.

Terence McKenna once pointed out that virtually all useful research on psychotropics is carried out illegally and is often largely funded from underground. Legalization would make possible a much tighter control from above over all drug research. The valuable contributions of the entheogenic underground would probably diminish or cease altogether. Terence suggested that we stop wasting time and energy petitioning the authorities for permission to do what we’re doing, and simply get on with it.

Yes, the Drug War is evil and irrational. Let us not forget, however, that as an economic activity, the War makes quite good sense. I’m not even going to mention the booming “corrections industry,” the bloated police and intelligence budgets, or the interests of the pharmaceutical cartels. Economists estimate that some ten percent of circulating capital in the world is “gray money” derived from illegal activity (largely drug and weapon sales). This gray area is actually a kind of free-floating frontier for Global Capital itself, a small wave that precedes the big wave and provides its “sense of direction.” (For example gray money or “offshore” capital is always the first to migrate from depressed markets to thriving markets.) “War is the health of the State” as Randolph Bourne once said—but war is no longer so profitable as in the old days of booty, tribute and chattel slavery. Economic war increasingly takes its place, and the Drug War is an almost “pure” form of economic war. And since the Neo-liberal State has given up so much power to corporations and “markets” since 1989, it might justly be said that the War on Drugs constitutes the “health” of Capital itself.

See Also:
Arguments For And Against Drug Prohibition
Legal Issues Of Cannabis
Cannabis Rescheduling In The United States