Think it through completely ~if you collapse your knees into the dirt you lose agility.
Spot-on correct. ^^
Throwing from a knee is a good thing to be working on, but receiving on a knee (or knees) will burn you. It's an unbalanced and unathletic position to use when receiving a pitch. You reduce or even lose the ability to shift laterally to block an errant pitch, and with a pop fly around Home or a bunt, it's harder to get up and on the ball.
I don't know why anyone would teach that. It's seems akin to the thinking that diving into 1B gets you there faster than running.