Among many good reasons Richardson offers, perhaps the best:
Because of the ongoing militarization of our police forces, putting police officers in SWAT vests and tanks, often granted on the basis of small drug arrests, which creates a perverse incentive to increase more pointless arrests. Radley Balko has done the best reporting on this at the Cato Institute, Reason magazine, and now the Huffington Post.
See, Why Legalize Marijuana – Best Reasons to Legalize Marijuana – Esquire.