Decriminalization sounds good in theory, but the "war on drugs" is a multi-billion dollar money making machine that feeds our growing police state, and face it, the American populace is a statistical paycheck, taking marijuana out of the picture would be equivalent to telling thousands of police officers they can kiss their retirement goodbye. Many (if not all) police drug interdiction groups use drug money as their supplementary budget, to buy cars, guns, body armor, etc. The surplus goes to line pockets in the form of bonuses, benefits, 401k, and so on.
Then there's the prison system, which benefits from the free, inexhaustible laborers that the "war on drugs" provides. And the tobacco industry which doesn't want the competition, the same goes for the textile industry.
These organizations, local/state drug enforcment, DEA, big tobacco, DuPont, etc, are not going to give up easily, and decriminalization will only delay and possible benefit them. I say it may benefit them because decriminalization will not stop some of the more egregious problems with the drug trade and that could be used to re-criminalize marijuana down the road, gaining us nothing. No, when it comes down to it, legalization and the dismantling of the mechanisms that are currently in place to enforce these laws, removing the authority of these agencies is the only way to eliminate their influence in our political system and win back the right to decide what you'll do with your body, with your property and with your life.
In my opinion, your dealer doesn't want legalization because he runs a criminal enterprise (not dissing anyone) and makes money from the risk involved with providing a prohibited substance. His profit margins are at stake, and he's feeding you disinformation to justify it.
Even if the government taxed the hell out of marijuana cultivation, we'd have our foot in the door, public opinion of marijuana would slowly but surely change and with that change we'd see more tolerance, and the ability to sway laws, tax or otherwise, in our favor.