Let me add another comment here, at the risk of repeating not only myself, but I'm sure countless others who have undoubtedly made this same point:
Just what (in a phono cartridge) do we think we're going to demagnetize folks? It would have to be made of ferrous materials, right, or it couldn't become magnetized in the first place, right? In a MC cartridge, the only ferrous materials are the pole pieces and the magnet(s). I can't imagine why on earth anybody would want to demag those! You want them to be as powerful as possible!
If the armature (that little cross- or square- shaped thing at the end of the cantilever that the coils are wound on) were made of iron (like in a transformer or solenoid) it could become magnetized. But using a ferrous material in that location would ruin the performance of the cartridge due to the electrical choking action of the metal core.
I think currently there's a high-end cartridge which has an armature made of mu metal, a material whose primary characteristic is its ability to shield things from magnetic fields. But most armatures are made of synthetics which are more compatible with the cantilever suspension materials against which they're compressed.
Let me further add that the fluctuating magnetic field that could be developed around the cartridge coil windings by sending a small alternating current through them would be too weak to do much of anything, good or bad.
