That's a fair question, there are multiple reasons both for work and personally. I won't go into the work reasons, except to say that a lot of my companies clients ordered XP with the machines they built (these are industrial systems for everything from manufacturing to government to scientific...you name it. I get calls (1 or 2 a week) from customers reporting the issues as described in the link I posted for reference after they upgraded the systems to SP3.
On a personal level, again multiple reasons.
1. Some of the software I must run *require* SP3, they won't even install without it, I'm talking applications, some local, some remote.
2. Running into problems with 2 other programs I am trying to run (1 database related, and a game my daughter plays), that randomly lock up while they run. There is seemingly no pattern, and I have yet to determine exactly what the problem is. The machine locks up completely, screen goes blank, the whole nine yards. Because of this, there are no system logs etc. that I can refer to to attempt to glean any info.
Everything, and I mean everything else is %100 up to date (bios, drivers, etc).
I had previously experienced the problems described on another machine running SP2 and after installing SP3 (which luckily caused no issues, although there were some corrupt entries, I cleaned them out manually as there were only about a dozen) they were gone. Point is, SP3 solved the problems. So I am hoping/guessing doing the same on the machine in question will work as well.
3. Somehow, a patch was applied to this XP machine, a patch intended as a hotfix for SP3...even though SP3 had not been installed (don't understand how this can be, I would have thought it would refuse to install but whatever). Although I was able to remove it, it left remnants in the registry. I'm pretty good with working in the registry but not that good, so again, hoping the application of SP3 will resolve that as well.
Hope that explains it.
If CCleaner does NOT address something like this, I'd like to suggest an option to scan for, and clean entries in the registry based on user entered masks, with the ability to list all relevant entries, and be able to edit/delete en masse. Definately an advanced option, not sure if Piriform will go for it but, it would be VERY useful.