Again, it is no more "nannyism" than the current laws enabling the FDA, or laws against speeding, or laws against any number of other things that can be considered a danger to public health. The government has a responsibility to protect its citizenry. Decriminalizing pot, is FAR from being a "nanny" in my opinion.
As for how decriminalization would work, I don't care if someone sells you a plant, as long as they don't have a plant store, or sell them at Home Depot, or set up an Ebay store! If Joe Blow down the street wants to give you a pot plant, I don't care, and it would not be illegal! My motivation is not to keep you from smoking or growing pot, or even to keep someone from selling it to you, just not as a legal commercial enterprise, endorsed by the government.
If you want to see what will happen to pot with that plan, take a look at cigarettes! I would venture to say, if cigarettes were grown and produced naturally at home, they wouldn't contain 576 chemicals designed to make them more addictive and desirable, and they probably wouldn't be nearly as deadly. It is the commercialization of smokes, and the government entanglement with taxation, which has created a social problem we seem to be stuck with, in spite of millions of deaths each year due to lung disease!
Who's to say that in 100 years, we may discover that daily habitual use of commercially produced pot is just as deadly and dangerous as cigarettes? And we still have to deal with the issue of second-hand pot smoke, and how it effects those who don't smoke pot! Their rights to have clean air to breathe is still in play, and we have the same dichotomy as we currently have with cigarettes.
Decriminalization will relieve the restrictive governmental attribute, and enable those who wish to smoke it and grow it, to do so in private, without fear of legal reprisal. It will also pave the way for 'medicinal marijuana' and pharmaceutical grade production in a controlled way, to be prescribed by doctors and monitored by the FDA, like all other drugs. What it doesn't allow, is governmental duplicity and culpability in endangering public health, which is what government is supposed to be all about in the first place.