It isn't duct tape, but I do have electrical tape on the power cables of both my '82 Electrolux 502S and HOOVER 119 where the insulation has split but the internal wires are still fine. It doesn't look as clean as it would with a replaced cable, but I'm not a staunch restorer and I actually think that a few imperfections here and there give vacuums character, so long as they work and are safe.
Oh, almost forgot - my '96 Electrolux Contour has black duct tape on the hose end that the previous owner put on as it had cracked and was falling off. I probably could remove it and fix it with glue, but it doesn't look too bad (the tape is the same colour as the plastic) and it works, so why fix it if it ain't broke!!