Im not sure that's true either, as in tobacco smoking just 1 or 2 a day can be pretty much the same thing to a lot of people.
If you smoked 1 or 2 a day its not the same as having 1 or 2 drinks a day either. For a start 1 joint a day will stop you from driving a car or operating any type of equipment. Im assuming that it will still be a crime to operate a motor vehicle with THC in your system. 1 good joint will prohibit you from driving or pretty much working legally for about 2 days.
If youre going to use Marijuana, grow your own and you will need to leave about 3 days before driving.