I'm inclined to agree with CA's
list of reasons for detecting XCP:
Installs without user permission, presenting only a vague and misleading EULA
Changes system configuration without user permission at time of change.
Defends against removal of, or changes to, its components
Silently modifies other programs' information or website content as displayed.
Includes mechanisms to thwart removal by security or anti-spyware products.
Cannot be uninstalled by Windows Add/Remove Programs and no uninstaller is provided with application.
Perhaps Spybot needs a separate category of "rootkit" for software that hides files or processes from the administrator of the computer, even if the software doesn't do anything else malicious. I'm inclined to believe that most people don't want rootkits on their computers, regardless of who put them there and why.