Zazen, I still don't think it would be hard to tax, nor do I think many people would wind up growing their own stash...
First of all, it would be much, much cheaper for me to roll my own cigarettes. I do not do this on account of the inconvenience, mess, hassle, etc. I have better things to do with my time. So do most other people who smoke cigarettes. It won't be any different with marijuana.
Second of all, even if the assets taken per raid are much more valuable than the crop itself, that does not take into account the fact that drug raids do absolutely nothing, zip, zilch, nada, to stop the flow of drugs in the first place. OK - *maybe* it causes a few hours inconvenience as one goes through the network shopping around, but on the whole, it solves nothing. This is because of the sheer volume of product being moved about that the government never lays its hands on. That's a lot of transactions that are not being intercepted.
Also, drug raids are sporadic sources of income for the government. Many completely fail and do nothing save get some police officers overtime checks.
OTOH, drug deals are non-stop transactions going on every hour of every day, all across the country. One year's taxation of these transactions is going to both be more stable than the raid income, and quite probably more profitable. Especially considering, once again, the "inconvenience motive" for people who aren't particularly industrious to go to a store and pony up slightly more cash than they would need to have a stable crop for their own personal use.
Of course the government can't admit this for two reasons. First of all, there's really no way to put hard statistics on it, since most people won't answer surveys. Secondly, once the true numbers came out, they would show plain as day that the government's "War on Drugs" has been hopelessly lost.