In my experience, the registry can and generally does become wildly messed up over time (just like Windows itself). Most uninstallers do not work correctly, leaving crap all over the place. A vanilla install of Windows XP leaves, depending on your definition of error, about 180 registry "errors". The other problem with the registry is that changes are not made in place: the original value is marked as invalid but remains in place, and the new value/key/whatever is appended to the registry file(s). Over time, this causes the registry file(s) to become bloated with junk, and information which is displayed in a nice hierarchical tree in regedit, for example, is actually stored all over the place.
I've used Registry Repair Pro (3B Software) for probably over a decade, and the only time I've ever seen it mess anything up is when I was being less than careful with its advanced features. The question of performance is answered easily by this example: My wife's desktop took forever to boot, like 10 minutes. There weren't a lot of startup items, and it was malware free (another discussion). I ran RRP on it, found about 1900 errors, fixed them, defragmented the registry files, rebooted, and it literally went from the Windows logo to full desktop GUI in about 20 seconds. I call that proof. I have seen this same performance increase on literally hundred of machines over the years. I currently care for about 75 machines where I work, and I probably run RRP on each machine at least once a month.
Besides these two main functions, RRP's ability to search for one or more strings I have found to be invaluable in uninstalling certain "valid" programs which resist removal; Software Updater is one example of this. Almost impossible to remove. You can remove the Roxio or Sonic suite that installed it, and it remains, and it's dug in deep. Without RRP, it would take way longer to find and remove the registry crap using regedit alone.
All that being said, RRP has a few minor bugs, is rarely updated (I think the current v4 is about 6 years old), and the publisher has been linked to some shady threatvertising tactics on a couple of occasions. If you buy a license for RRP, and use it carefully, it's worth the $19.95 (or whatever I paid for it).
IMHO