Myself and the vast majority of my colleagues think it is absolute idiotic and a waste of time to require a doctor's note for a sick day.
It benefits no one, wastes time and prevents people with more urgent needs from being seen in a timely basis, and increases the chance of spreading diseases that are better fought at home away from people.
It's not uncommon to ask people who are made to do this by their boss how long they personally feel they need to be out for medical leave. I have seen at least one place change their policy after a number of their employees were prescribed mental health days for full recovery.