Im glad to let you know my windows machine has been running for 4 years w/o re-install of windows XP, and its running fast and healthy now (well, I do consider myself a power windows user.

) Windows, as an OS doesn't need to be re-installed every 6 month, but users do need to know how to maintain it, such as running msconfig, cleaning application data, cleaning registration data, running free AV softwares, etc,
Thats not saying that OSX need no care, trash files and residues of deleted apps (drag and drop to trash can? lol, not clean) are always there, and sometimes weird disappearance of disk spaces, etc,
For re-install of OSX, only at the beginning when u get the new machine, you can choose to do so to save alot of disk space. otherwise, unless u don't have important data, I don't suggest re-install the OS.