The advantage of MI (moving iron over MM (moving magnet) is much lower coil inductance and lower moving mass. The result is faster transient response and wider bandwidth. Grado cartridges typically have coil inductance in the 10's of millihenries compared to the 100's of millihenries of MM's.
Grade Master3 cartridge--Moving Iron Design
Posting this because I just bought my first Grado cartridge since my entry level MM decades ago.I chose the Master3 because it was highly rated and I was intrigued by the Jarrah Wood, boron cantilever and the nude elliptical stylus at a $1,000.00 price point. It's not the most expensive cartridge I have mounted on my VPI Signature table, but it has an excellent balance. Have run it about 20 hours now and it has settled in.
My question is the moving iron system is an excellent design and why haven't more manufacturers seen fit to utilize it?