Actually, the Grados are Moving Iron (MI) - a variation on the MM theme. A Moving Iron cartridge uses a fixed coil and fixed magnets. The stylus is attached to a small piece of magnetically permeable metal (such as iron) located in the magnetic circuit between the coil and the magnet, but not to the magnets themselves. When the stylus vibrates so does the permeable metal, which in turn disturbs the magnetic field around the cartridge, which in turn generates a voltage in the coil.
The advantage is that the permeable iron can be much lighter than the magnet, yielding many of the same theoretical advantages of the moving coil.
The advantage is that the permeable iron can be much lighter than the magnet, yielding many of the same theoretical advantages of the moving coil.