1. No one, not the American Medical Association or the courts, has scientifically proven pot has medicinal benefits.
The debate over its medical value is too highly politicized to go into here. However, just because something does not have a proven medical usage is no reason to arrest people who use it. Tobacco has no medicinal benefit, yet it is legal.
2. Marijuana's addictive.
We could also argue whether it is or not, but so what? Tobacco is addictive, yet legal.
3. It's a gateway drug.
The old logical fallacy "if before then because." As George Carlin said, "Mother's milk leads to EVERYTHING." Why are only the illegal drugs considered gateways? Sugar, beer, and caffeine are never cited as gateways, even though most people are exposed to them before they get to illegal drugs.
4. Marijuana use and possession would still be illegal on the federal level.
True. But no reason to make state officials complicit in the insanity.
5. Three ounces is not a small amount of marijuana.
There is no legal limit on the amount of alcohol or tobacco you can own, so why should there be any limit on the amount of cannabis you can own? Well, to ensure people aren't selling it, of course. But is there really still a thriving black market for alcohol, or did the 21st Amendment put an end to that?
6. People aren't being arrested for possessing small amounts of marijuana anyway.
However, the police still have the power to arrest someone for it. People who smoke marijuana have a negative attitude towards people in the law enforcement profession, because they see them as the enemy. If the war on marijuana was over, the police would have a lot easier time interacting with the public.
7. It would still cost law enforcement a lot of money for prosecution of such things as driving under the influence, public use, selling or giving to children and others.
We ended alcohol prohibition, and the police still have to deal with people driving under the influence, public drunkenness, minors in possession, etc. But they no longer have to deal with catching bootleggers and investigating gangland wars over liquor turf.