In my current workplace, an employee's supervisor reserves the right to ask for a doctor's note if the employee is out for three days or longer. And as an HR rep--I basically AM the HR department--I recommend supervisors ask for a doctor's note anyway, regardless of how long the employee is out. There are still people who game the system (e.g. accrue a day of leave then immediately take that day), but the doctor's-note policy cuts down on sick-leave abuse.
As for the people who do game the system, that's something I just. don't. get. And I agree they're being SS's. In the spring of 2006, I became severely ill and had to go on medical leave for two months. I had three months of sick leave saved up. My illness is under control now, thankfully, but could flare up again at any time. That's why I'd rather have the sick leave on the books and not need it, than need it and not have it.