If you regulate and control the availability of cannabis you do exactly that, take control of it and remove it from the unregulated illicit drug market. There is an argument to say that government can produce cannabis and sell the taxed product cheaper than the black market without giving it away at a ludicrously cheap price. The taxes go back into government coffers, ideally into the social and health related initiatives that government would provide to help deal with cannabis related harm.
The benefits, firstly you would not have an enormous amount of money spent on policing and the subsequent issue of police corruption, with no black market you effectively cut out organised crime. In addition to this, licensed growers just like licensed tobacco growers would have a set quota of how much and what varieties they would grow. Research would focus on ideal cannabis strains of regulated THC strengths, ideally controlling the unknown variables of the current illicit cannabis market and the multitude of varieties and strains currently available through the blossoming illegal horticultural cannabis industry. Effectively capping strength and varieties of cannabis.
Ideally there would be a provision for home growers to grow a certain amount of cannabis for personal use and you could only buy so much at a set time from licensed vendors. The difference between the control and regulation of cannabis and for example, the sale and promotion of alcohol would be significant. Whereas with alcohol we are bombarded on a daily basis with unbridled advertising, ideally there would be strict regulations regarding the promotion and sale of cannabis. No big breasted, hot looking women on prime time television promoting cannabis or football teams being sponsored by cannabis companies.
And yes, there will be individuals who experience problems associated with the use and abuse of cannabis. There will always be issues associated with the use of cannabis regardless of the law. The question is how would you rather purchase cannabis; through a regulated market or an uncontrolled market of organised crime?