Taxation is not the justification for legalizing marijuana. It would be better for the government to make money from the industry than to blow a fortune trying in vain to keep up the ban, but there are plenty of good reasons to legalize marijuana. One of the biggest is to cut organized crime out of the picture.
Legalizing it and allowing people to grow some plants but no sales would be preferable to what we have now, but it would not be the ideal situation. Most pot smokers will not go to the trouble of growing their own. They won't have the time or the space or the skills required. Maybe they're lazy or just don't feel like messing with it. Most don't smoke enough to justify the hassle of it.
The Dutch are allowed to grow up to five plants each and in medical marijuana states people can grow their own, yet the coffeeshops in the Netherlands and the medical marijuana dispensaries in California do gangbusters business selling expensive pot to people who could just grow their own. If people were allowed to grow their own, there would still be a substantial black market. Homegrowers would in many cases sell part of their crop, and organized crime would still be selling a lot. Home growers would probably cut into organized crimes business some, it's just hard to say how much though. Dealers would probably deal with them for the most part because they would always have a reliable supply whereas homegrowers would be hit or miss and they'd have to deal with a lot of them to keep a steady supply for their clientele.
Then we'd still have powerful organized crime groups making lots of money from pot and pushing their far more dangerous drugs through the people selling their pot. Mexican Drug trafficking organizations today make most of their money from pot and they supply 90% of the cocaine and 80% or better of meth and heroin consumed in this country. They piggy back the hard stuff in on top of the marijuana and much of it ends up being sold by the people who sell their pot.
Legalization and regulation similar to alcohol regulation makes the most sense. Competitive commercial production and distribution of marijuana in a legal environment would drive prices way down. Sure the government is going to tax it, but they could put high taxes on it and still have the price be below what people are paying today. They could kill the black market for marijuana, which is most of the black market for illegal drugs, and make more than enough money to pay for the cost of regulating the industry and the continued efforts to do something about the problem of the hard stuff. Just letting people possess it and grow a few plants would leave us pretty much in the same boat we are in. It wouldn't be “problem solved.”
Amsterdam cafes rely heavily on tourist dollars and Mexican pot is not considered high quality; Hydro is superior. Creating a regulated industry will also cause damage to our culture ala Holland. The Dutch had second thoughts and tightened their drug laws: http://www.freerepublic.com/focus/f-news/2149385/posts FYI, there is a huge black market now, giving people the right to grow their own would reduce it, lazy defense aside, and reduce the demand for foreign sources. It's amazing that people profess to want personal rights, but would rather have the government get more involved in regulation of those rights instead of just taking personal responsibility for them and keeping the government effectively out of them.