I said "fully understand". There are for example COM registrations in the registry where multiple parts link to other parts, and it's very easy even for a COM developer to mess up manually trying to set or reset a configuration. As someone who created installs for a while and worked on an organization's installer package before that, you haven't the foggiest how to safely clean up after a POS app fails it, because you don't know what configuration the installer did.
Ah. Good point.
The bottom line is don't run dodgy software, like A/V or open source or other known generally unprofessionally-done categories of things, on a system you care about.
Some would argue that A/V software is unnecessary or even questionable. For a Windows system anymore, though, it has become more of a necessity, not because Windows is any less secure but because Windows is far and away the biggest target. Purposely not running A/V software on a Windows box is just asking for trouble unless you never go online and never load in external media like CDs or USB storage devices.
What registry cleaners are for is people who don't treat their system sensibly.
On this point I disagree with you. Every piece of software I have installed under Windows, from open source freeware to applications costing several hundred dollars, never removes everything it dumps into the registry. There are always traces left behind, usually stupid stuff like file associations but often more important things like DLL associations. This is not limited to little fly-by-night companies either - I have traces of Microsoft stuff in the registry that is no longer on my system.
And, what about all the OEM junkware that comes with a new system out of the box? The registry is already full of crap before the system even gets powered on.
There is another choice.
There are always choices. Windows is what I use for recreational computing (surfing, gaming, writing, etc.) but my serious stuff happens on my Linux installation.
Because I use Windows to access the internet, I use A/V software. I received a version of BitDefender with a 2-year license to evaluate. I was unaware that the uninstall is even more unclean than most, though the crown probably goes to Norton as the most difficult to clean out. NIS is famous for preventing other packages from installing, even after a full uninstall, which is the reason I will *never* install it.