Spontanoulsy I'd say not only eye candy. A mouse cursor indicates that the program is busy, and thus you can not make any commands. A throbber indicates a progress, but you can still work in the program.
From Wikipedia: "Normally, it is possible for the user to continue interacting with the program while the throbber is animating (one such possibility may be to press a stop button to cancel the action that the program is doing)."
For instance, when thumbnails are created in a folder, you can still interact with XnView. However, it would be very nice to have a throbber in the tab of the thumbnail pane, just like in Firefox, when loading a site.
I wouldn't like to see a big throbber in the middle of the screen or so, that would be too much, something more sophisticated like in the Firefox example.