It's true that Windows is constantly modifying its registry, mostly for what the system people used to call "housekeeping" - but most of these modifications have little long-term significance, and some have none. It has happened many times that my registry was seriously damaged, usually by installing some third-rate software or by something that happened during a power failure; sometimes it was so badly damaged that the system wouldn't boot at all. I fixed it by restoring the registry, sometimes from a backup a few days or a few weeks old. Sometimes some silliness like a program's recently-used-file list disappeared in the restore, sometimes some minor changes in system or application configuration disappeared, and sometimes an entire app which had been installed since the last backup had to be reinstalled.
I would rather have a recently-used used file list disappear, or even have to reinstall an application, than to have to reinstall Windows and almost all of the applications, which is sometimes the alternative if the registry is badly damaged.
If you are very worried about the problems of using an old registry, you could back up the registry before every software installation, and before almost any major change in system configuration. There are also ways of backing up the registry on a schedule, e.g., every night.