Registry: Defrag vs Clean

For Vista or newer:
Registry Defrag, is it good or bad?
also contains link to  Registry Cleaners are good or bad

Because the registry defragmentation process enables you to remove empty registry keys, you must defrag the registry after you have performed any activity that removes a lot of entries from the registry.

For instance, after you clean up the registry using the registry scanner tool, all the invalid keys removed leave behind their empty place holders. In the same way, empty keys may get left behind when you uninstall applications from your PC. So, even if you would have removed all unwanted information from the registry—or uninstalled an application, the size of the registry may remain the same due to the existence of these empty registry keys. This is the reason you may always defragment the registry after registry scan and repair and program uninstall processe

Source:
Why You Should Defragment Your Registry
http://maketecheasier.com/why-you-should-defragment-your-registry/2008/01/29
Note to self:
Defrag=Compact=Compress

Related articles/tools:
Compacting the Windows Registry
http://erpman1.tripod.com/current/regsmall.htm