It depends. If you have a large, collapsed folder tree and want to 
drag into a few-levels-deep folder, maybe even scrolled out of view, 
that ain't a task for people without fine motor skills either. The 
benefit of the context menu solution would be that users won't have to 
keep the left mouse button pressed, which is an additional challenge 
for some.

I am not for this idea.  Especially for users (like me) with 100's of 
mailboxes, copying/moving via context menus would be very unwieldy.   
If anything, it is *more* difficult to maneuver through the several 
submenus than it is to drag/drop.

I have had a number of (dynamic interface) users request the ability 
to copy or move email messages without using drag-and-drop.  Many 
other IMAP clients offer "Copy to" and "Move to" options in the 
message context menu and/or the same as a drop-down selection list 
from the toolbar.

From looking through the JavaScript code, it looks like the work 
would be mostly in building the folder list display, with the action 
logic being about the same as in the folder drop handler when messages 
were dragged.