It usually isn't so much a "I need to reformat because of this issue"; it's usually "Windows has accrued enough cruft that I could benefit from starting anew". I suppose you could call it OCD; I'd call it a sanity check. I find developers and gamers who install lots of beta software or are "frequently" installing and uninstalling software suffer from the poor system of installation of applications on Windows. Windows installation wizards pretty much throw files wherever they please and uninstallation wizards almost never actually remove everything. There are plenty of applications like Revo Uninstaller which are meant to help clean up after uninstalling an application, but there is no silver bullet. Say you do encounter an issue, having a clean filesystem/registry means you're more likely to be able to reason about your system. For everyone else, this problem has pretty much been solved by package managers in free software and a combination of the AppStore, homebrew/macports, and ".app"s in OSX.

It depends really what you do and how much time you spend in Windows because that's what determines how quickly the cruft accrues. If you've ever uninstalled software, I can almost guarantee that your already poorly organized Start Menu has a few folders and shortcuts that are empty/broken.