MM cartridges and capacitance

Can someone explain to me why an MM cartridge would "want" to see more than the minimum possible level of capacitance loading?

This question is provoked by a lot of commentary on "that" "MMs are great" thread, and a question someone just asked, and the fact that I have two phono stages (granted, older Japanese stages) which have multiple capacitance settings and I have never gotten a "better" result from being at the high end rather than the low end. In many cases, it doesn't seem to make a difference, but so far I have not gotten an improvement from raising the capacitance setting.