ArchiCAD 9 Help says this about Drag, Rotate, Mirror a Copy:
ArchiCAD 9 Reference Guide > Techniques > Creating Element Duplicates > Drag, Rotate, Mirror a Copy
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This feature works in both the Floor Plan and the 3D Window. In the Section/Elevation window, the feature works on both additional drawing elements and Doors and Windows (dragging only). In Detail Drawings, you can use it on any drawing elements.
I think the reason you can create copies of only Doors/Windows in S/E Windows but not any other 3D Construction elements is because in the S/E Windows you are not capable of defining the X,Y coordinates of the duplicate the way you can do it in 3D.
But I agree this could be more subtle. Like this. If you drag a copy of a Library Object, if will be moved in a vertical plane parallel to your Section Line. So with such limitations, but it still could be done.
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