Not only doctor's, but any other professional or people with specific skills.
I knew of a Software Engineer that has a corporation where he and his wife own all the shares. He is the only employee of the corporation and does not take a salary.
Once a year the corporation kicks a big dividend to the shareholders (husband and wife) that is eligible for the dividend tax credit.
Other than the 10% corporate tax, the husband and wife pay very little tax, enabling them to not pay the same amount of tax a regular salary employee would pay.
Furthermore they write off an amazing amount of stuff through the corporation allowing them to avoid GST taxes as the corporation writes the GST off against what it collected.
If Revenue Canada were to audit your friend, the good times would be over.
That's why these taxation measures are so silly. The government is failing to audit enough individuals, so therefore "everyone" must be up to no good.