> GIMP has Script Fu (and a newer Python interface) which has been used for wonderfully time-saving things (e.g. process a thousand images with the same plugins and settings and such, implement your own custom processing tools by combining existing ones, etc.)

Begin scriptable and having a CLI are two orthogonal concepts. With Script FU, GIMP itself executes scripts. If gimp had a CLI it means that you could be able to do stuff like "gimp --select-pen 2px solid black --moveTo 0 0 --lineTo 100 100 --lineTo 50 50" for instance (for which I don't see the point of using GIMP to do).