Chakster, You seem to use the term "impedance" interchangeably to talk about both the voltage output of a cartridge (usually expressed in mV in relation to a stylus velocity of either 3.54 or 5.0 cm/sec) and its internal resistance. Thus it is not the low internal resistance/impedance of an MC cartridge that necessitates a SUT so much as it is the low signal voltage that these cartridges typically generate. However, in a way you are right, because the low internal resistance makes an MC cartridge more of a current generator than a voltage generator; the SUT merely serves to trade off current for more voltage.
As to this feature of auto-loading, thanks to Totem for reminding me of some of the other now vintage products that once offered this feature. I think most of us favor choosing the load to suit our musical taste. I prefer a phono stage that allows me to select the load resistance, and for MMs, the load capacitance, most conveniently. I've given up on the purist approach that says no switches in the signal path, but if I have determined what load R and C I prefer, I might then hard wire the parts in circuit, bypassing the switch.
As to this feature of auto-loading, thanks to Totem for reminding me of some of the other now vintage products that once offered this feature. I think most of us favor choosing the load to suit our musical taste. I prefer a phono stage that allows me to select the load resistance, and for MMs, the load capacitance, most conveniently. I've given up on the purist approach that says no switches in the signal path, but if I have determined what load R and C I prefer, I might then hard wire the parts in circuit, bypassing the switch.