Never - although not intending to a adopt a holier than thou attitude, I've only worked in positions where unprecedented sick days lead to a struggle for my colleagues, or required work will not get produced when promised thus causing greater problems than going to work in the first place.
Nonetheless, I absolutely detest the UK employer's commonplace attitude of going to work when you are genuinely ill! You are far better off taking the time off to rest/recover swiftly than struggle into work, risk colleague illness, working at sub-par standard and then taking twice as long to get better!