Why would you need to store the same ton for years. You could swap out tons every year.
And now, why would you need to make a weed backed coin? What would be the purpose? Tomatoes are valuable. Should we have a tomato backed coin as well?
There is a big difference between gold and weed. Each ounce of gold is interchangeable. The same is not true for weed, or tomatoes or oregano or pork bellies. (By the way there is a market for pork bellies on the Chicago Mercantile Exchange.)
A marijuana backed altcoin could be an interesting experiment as it could provide a stark contrast to precious metal backed fiat. Storing tons of marijuana in a vault for years could provide greater price stability than swapping out weed annually, that would be an idea behind longer term storage.
A weed backed coin could be something like a bet on the long term value of weed. If market expectations were higher weed taxes, reduced weed production and other policies which could increase the value of weed per ounce. Storing some in a vault with the expectation that it will appreciate in value could make for a better currency backing than gold. Like for example, let's say taxes on legal marijuana were raised and production was lowered which made weed 20% more expensive over 5 years. That appreciation in value could make for an interesting contrast to fiat or gold.
It could be compared to tobacco and alcohol excise taxes where cigarettes used to be much more affordable, when taxes were lower. Storing tons of tobacco in a vault while the price is appreciating could make for a cool experiment. The same might be said of weed.
One reason for experimenting in this way is, it could capture peoples imagination and encourage them to experiment more in ways which could lead to greater understanding and innovation in terms of how markets/finance function, which could benefit humanity over the long term. That's the main motive for me. I don't even smoke weed lol.