I get the impression that many Linux users have specific old versions of software (often a window manager, which complicates the issue) that they want to keep using forever, but want to simultaneously use the latest version of some other software (like a browser).
Linux is pertinent here because we're talking about X11, but it should be noted that this is by no means specific to Linux - the same could be said about any OS with widespread use by different groups of people.
