Please tell me you're kidding! Currently, Windows XP is the dominant OS in the personal computer market. XP: 61.54%, Vista: 24.35%-J-Mac
Percentage of people using a given OS does not dictate how modern it is, but rather its age.
So why - if you agree that XP does indeed require registry cleaning at times - does the age of the OS even come into play?
Because the age dictates what kind of optimizations have been put in place at release. It's common sense that Microsoft will learn tricks as time goes on and will make improvements to their code.
If the vast majority of users are still using XP then registry cleaners are certainly a needed commodity - still.
Now what are we talking about here? Registry cleaners or tweakers? There's a difference in functionality there. Tweakers make actual changes to existing values in the registry while cleaners remove the flotsam and jetsam no longer needed. Regardless, registry tweakers and cleaners are certainly needed for those who are still running Windows XP...but not those running more recent operating systems.
My entire viewpoint is based on not how many people are still using Windows XP, but its age. It's still a fine operating system, to be sure, but it does not have the optimizations and refinements of later, more recent OSes.