It totally depends on how you use your PC. Power failures and improper uninstalls are good examples of things that can wreck your registry. I can think of a few instances where I've done something, forgotten about it, done it again slightly differently and it hasn't worked which has been cured by cleaning the registry, so the damage is not always noticeable and might come back to cause problems days, weeks or months down the line - that's why I personally run CCleaner semi-regularly to help prevent that.
Then again I work in the maintenance department of a large museum, a large part of the job is preventative maintenance. "If it ain't broke, don't fix it" isn't a phrase we tend to use! Our philosophy is that things will break a lot less frequently if we keep them fixed. A lot of our PCs are completely locked down but we still keep an eye on registry errors because stuff can go wrong at any time and for any reason.