I can confirm this feature would be useful (for me at least 🙂 ). I have files usually tagged with 5-6 tabbles, and for me consistency it quite important. Often, I find myself opening Tabbles on the source file to see how a file is tagged, and redoing the same tagging on a new file of the same type (on a new incoming electricity bill for example).
So, how I see it:
1) in the Tabbles GUI or windows explorer, select one already tagged source file
2) select Tabbles menu item ‘Copy tabbles…’ (or in windows right-click menu)
3) (icing on the cake) popup opens and shows existing tabbles on the source file, allowing to (de)select some tabbles.
4) select one or more target files
5) select menu item ‘Paste tabbles’ (or in windows right-click menu)
6) if at least one of the target files is already tagged, then open popup: “do you want to replace or merge existing tabbles <list tabbles>”
7) replace or merge as selected in the previous step (or simply tag if there were no tabbles on any of the destination files)
In a different domain, this is similar to how Adobe Lightroom 4 is handling copying parameters from one photo to one or more others.