I'm not sure which version of Ccleaner you've been using there "markku & gfinms" but the ones I've been using over the last 6 years have NEVER messed up, slowed up, locked up, or in anyway degraded the performance of any of mine or of any of my business clients computers that I do maintenance/upgrading on. Granted the performance improvements are sometimes small, but if you have somebody who is short on RAM and your trying to limit the amount of temporary files you have cached, clearing them out regularly helps to significantly improve web browsing times.
Ccleaner is also a lot faster at removing all of your history, temp files, etc. than the tools that are built into IE/Windows.
"removing a few dozen keys amounting to a couple of kilobytes" ....hmmm, last night when I finished my evenings use of "web surfing" ( roughly a 2 hour session ) I ran Ccleaner and even though I have tweaked some of the settings , it found 365,919,052 KB of temp files from Internet Explorer in 17,426 files , plus 650 MB of more junk I was leaving behind and removed them all in 4.893 seconds. ( this info was copied/paste from my log files in Ccleaner ).
I've even used the Registry cleanup tool in Ccleaner and its never degraded my systems.
I'm not sure of who you two might work for that your trying to "dog" Ccleaner so badly, but I say your analyst of the Software is way off base.
(NOTE : I don't work for Piriform either )
....and even if you can do a lot of the same things manually thru internet options/regedit/msconfig etc. etc. , the convenience of Ccleaner make it a "must have " for any IT doing computer maintenance/upgrading.