Right now, dragging a file with the left button moves (within the same disk) and copy (to a different disk), dragging with the middle button does nothing. Other file managers (windows explorer, gnome nautilus, ...), if you drag with the middle button, show an options menu to chose whether to copy, or move the file. Adding this could be useful to move between disks and/or copying between folders of the same disk.
Are you sure you don't mean the right mouse button? It's working on Xubuntu 13.10 (with Xfce 4.10 / Thunar 1.6.2). I tried middle mouse button drag/drop on Windows 7 and it didn't do anything, right mouse button does. The functionality for right-button drag/drop was reintroduced with this commit: http://git.xfce.org/xfce/thunar/commit/thunar/thunar-standard-view.c?id=bb96457d8cf1a83052197b1e722eda3c59eb7433
You are right, it was my mistake. I was tinkering with the behavior of middle button on media player and somehow mixed my ideas. Tested it right now and it's working as you said. Sorry.