My argument for relegalizing crack is that by doing so we'd reduce the following effects of criminalization: deaths of innocents in drug-turf wars; deaths of users due to impurities or unexpectedly high potencies; enrichment of criminals; corruption of the justice system by enriched criminals; and lessened respect for the law in general.
Not to mention upholding the principle that adults should be free to make their own non-rights-violating choices (even if the choices they make are stupid).