The poll is missing an option: "No, I just edit the registry myself."
Registry cleaners are useful because almost all programs don't remove all their registry strings when you uninstall the programs. A registry cleaner/tracker can determine exactly what was written to the registry after you install a program. Although the leftover strings doesn't consume much space, it all adds up in the end.
Registry cleaner publishers usually claim that their products make your computer safer and faster, but this isn't entirely true. You can make your computer faster by removing any unnecessary or unwanted start up programs in the \LOCAL MACHINE\Software\Microsoft\Windows\Current Version\Run registry key yourself. (Back up your registry first.)
As for the "safer" part, Active X controls and some programs/drivers register themselves through the registry, but the registry cleaner/tracker can't tell if the code is of a malicious nature, so this is where an anti-virus product comes in.
Also, registry cleaners can't tell if a registered program is still needed or not, so there is some danger to using them.
The risk is like removing shared .DLLs...When you uninstall some programs, the uninstaller might ask you if you want to uninstall some "shared" files but the problem is that you don't know if there are any other programs which need those files.