In a similar vein comparing marijuana to alchohol, I'd like to see a law that would allow the growing of weed for personal use that parallels the law governing home wine-making.
You can freely make your own wine, with no governmental intrusion whatsoever up to a preset limit. Let's say 100 litres. (Not sure of the allowed amount.)
You can drink it, invite your friends over to drink it, your friends can bring some bottles of their wine over to share with you, you can trade bottles of wine between yourselves and discuss your home wine-making techniques to your hearts content.
You just can't SELL a single drop of it.
I believe the Canadian government in 1973 flew up a trial balloon suggesting exactly this, with a limit of 7 plants per person, but it never made it to the Commons for debate.
If pot-smokers could grow for personal use without fear of a bust, could share and trade their personal stash with friends, to whom would a dealer sell? Sure, there are still bootleggers of alchohol, and there would still be some dope-dealers, but I think it would kick the crap out of a particularly seedy, questionable black-market industry.
On the upside for the taxman as well, the home wine-making supplies business is quite large and healthy, paying their sales and business taxes like everyone else. I can see no reason to doubt that a similar small industry for hydroponics equipment would also flourish.