In Windows XP (dunno about other versions), you can't access the private registry keys of other users unless you're an admin with sufficient privileges and you specifically load the registry of another user into regedit. HKEY_USERS does not list every user of the PC, only the current user. The keys with short names like S-1-5-18 are special service accounts, not user accounts.
Therefore, it's unlikely that others can/will read your registry keys if your PC is secure. If your PC is not secure, then others can read your files and everything else, so Plus! is the least of your worries.
Also, it's not like Plus! is the only program that stores private data in the registry.
What about HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\TypedURLs? Every URL you've recently typed into Internet Explorer's address bar!
Or HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Player\RecentFileList? Every video file you've watched recently!
So, STFU.
/ 
