The thunar-volman "Removable Drives and Media" shows up in the GNOME settings menu, and possibly in others, too (KDE? LXDE?). However, this is a thunar bug: in the file /usr/share/applications/thunar-volman-settings.desktop, the line OnlyShowIn=XFCE; should be added to indicate that this is a XFCE-specific item.
*** Bug 8907 has been marked as a duplicate of this bug. ***
I'm closing this as WONTFIX. Why would anyone install thunar-volman without thunar on a DE that has a filemanager and very likely a volman too? This change doesn't make much sense to me. Feel free to reopen, but better discuss this on the mailing list or via IRC #xfce-dev.
Your opinion is based on the assumption that computers are not shared by multiple users. However, our computer is used by multiple users. Not all of them use the same DE, therefore multiple DE are installed (e.g. XFCE and GNOME). The GNOME user don't want to see the thunar-volman entry, of course, but uninstalling XFCE completely is not an option (because of the other user who prefers XFCE). Note that the media settings of XFCE and GNOME are named very similar ("Removable Drives and Media"), which makes using it hard it both entries are shown in the menu. Conclusion: DE-specific settings shouldn't show up in other DE, in case there are multiple DE installed.
This is certainly a controversial topic. For some people it might make sense, for some it probably doesn't. Indeed thunar does not restrict itself to a certain DE. It is simply the default filemanager in Xfce, but can be used in other DEs. That's probably why that setting is not in there by default. Also I do understand your reasoning, but I wonder why in your case you don't simply patch this in locally? All in all, I'd prefer to discuss this change on the mailing list than trying to solve this here, so that more people can comment on this. Whatever the outcome is, I'll be happy to change it or not.