Most MM inputs provide a 47K ohm load. MC cartridges usually need far lower load impedances, in the 100 - 2000 ohm range. In additin to insufficient gain you'll get response anaolies due to load mismatch.
Having said that, many high output MC cartridges are designed to work with a standard 47K MM input and will not have a problem with gain or impedance match.
Having said that, many high output MC cartridges are designed to work with a standard 47K MM input and will not have a problem with gain or impedance match.