* The ability to move the toolbar buttons around by clicking and
dragging (perhaps when in a special mode, perhaps always).
I see no need for that.
* The ability to right click on a toolbar button and bring up a context
menu.
That does not require support for drag events.
* The ability to middle click on a toolbar button to perform a modified
action (say, find-file-other-window instead of find-file).
That does not require support for drag events.
* The ability to click-and-hold on a toolbar button to bring up an
extended menu.
That requires support for down events, but not for drag events.