Answer:  I think your doctor's recommendation is based on a myth. While it is true if we lived in an optimal environment with clean air and water, and food that came from healthy soil, we probably wouldn't need to take supplements.  That's how Mother Nature intended the earth to be.  However, for those living in the US, it's a very good idea to take comprehensive vitamin and mineral supplements on a daily basis because of the significant decline in the last fifty years of the inherent nutrient value of fruits and vegetables that are available to us.  

Source: " Changes in USDA Food Consumption Data for 43 Garden Crops, 1950 to 1999,' Journal of the American College of Nutrition 23:669-82, 2004.