Thomas: that behaviour looks like a bug. Are all of those volumes mountable by the user? The drive mount applet is only meant to show icons for the mount points the user can mount.
Note also that the applet is using the exact same information for the list of drives as Nautilus is. If the applet is confusing, then wouldn’t Nautilus’s “Computer” window also be confusing?
To help debug things, I wrote a little program to dump all the data provided by GnomeVFSVolumeMonitor:
Does the output look sane for you? In particular, are any drives or volumes marked “user visible” that should not be?