Do Mac owners ever have to reformat their hard drives and reinstall their operating system?

Ive reinstalled windows xp 3 times. The first time was an accident, the second time my themes were really broken and i couldnt click anything and the third time the internet connection dll s were bad. i dont have a mac, but it seems very good and solid. I think the easiest thing to do with windows is backup and reinstall once a year for normal maintenence lol!