It really depends on the extent to which it is "legalized".
The issue you mentioned regarding taxation applies to both tobacco and alcohol as well - you can grow your own tobacco and make your own alcohol. Many people who smoke marijuana may have no interest in growing their own - the same way many tobacco smokers don't grow their own. Although, the point could be made that marijuana is probably a much easier plant to grow.
Furthermore, legalization does not necessarily allow people to grow for the purpose of selling. For example, it is legal to make alcohol for personal consumption but illegal to sell it.