The primary benefits of legalizing weed are the end of the horribly racist and unjustifiable imprisonment of thousands of nonviolent Americans, and awkward columns from old white columnists. A good secondary benefit, though: it costs the police money.
Many police departments, as you may know, fund themselves in part with the proceeds of assets seized from drug dealers. (For a horrifying look at how that process can be abused, see here.) The Wall Street Journal points out today that states that choose to legalize weed consumption, sale, or possession are sure to screw their local police departments out of that seizure revenue as a result (cops in Washington and Colorado "expect to lose millions.") Huzzah!
Alison Holcomb, an American Civil Liberties Union lawyer who was lead author of the Washington pot-legalization ballot measure, said no taxes were earmarked for law enforcement "in part because we're making this substance legal. It doesn't make sense there would be an increased need for law enforcement."
Law-enforcement officials counter there will still be a need to police such things as driving while high, underage marijuana consumption and unlicensed growing facilities.
Driving while high, teens smoking weed, illegal weed growing facilities... all things that police have already been policing for decades. The only difference now is you are subtracting many other related crimes, such as possession. Therefore, as the nice ACLU lady helpfully pointed out, there will be less crime for the police to police. Therefore they get less money.
Police, by contrast, argue that [Current crime]-[portion of current crime]= [More crime].