Let me start by saying that I am one of those types of Canadians who will tell you that smoking marajuana is not good for you. It is especially harmful to younger people with developing brains and perceptions. Regardless, let me also say that comparing the usefulness of pot to common garden variety vegetables and herbs is absolutely rediculous and grossly irresponsible. There is no comparison. One is a psychotropic, mind altering drug, the other is food. Give your head a shake my friend.
To add to the discussion of the underlying issue, I would like to point out that the real criminals of this world will continue to find alternative illegal methods of earning a profit should marajuana become legal. I am of the thought that our governments will never effectively remove this particularly inherent nature of humanity from society no matter how hard they try. To put it simply, some people will still find new ways to cheat the system at the expense of others.
If you say you want to grow 20 pot plants as opposed to the legal limit of 4 and you don't want the government interfering in your gardening business, I hear you and acknowledge your desire for privacy and freedom. My concern is that you aren't fully aware of what you intend to produce nor do I trust that you will merely use it for pest control. There really is only one known substantiated use for female cannibis plants which is to partake in it for medicinal/recreational purposes. Your comparison is not unlike saying that you wish to distill a large batch of high test moonshine to substitute for petrol when the price skyrockets... Any other justification is sketchy at best and some key people are wise enough to know this hence the proposed limitations on personal possession and plants.