Grains, Potatoes & Clay- Oh My!

I'm hearing a lot of recommendations to feed grain free food. All grain free foods have potatoes or montmorilinite (sp?) clay instead. Why would potatoes or clay be better than whole grains like brown rice, oatmeal and barley? What is montmorilinite clay?

IMHO I try NEVER to feed whole grains the digestibility is IMHO too low( cats dont need HIGH fiber ) ... I prefer if using grain to use something processed by a step or two and actually semi usable to the kitty ...Clay, I look for it NOT to be high in the list :it has a place as it is a cleansing aid ( ie less need for other ingredients )...I prefer sweet potato to regular for digestibility and nutrient content... potato( typical white/ russet type) can INCREASE blood sugars, potato protein is not going to do this but has its own issues like NO std definition as to what it truly is