It's purpose is to stop the daycare from teaching your pokemon moves as they level up, it could be a held item you give to the poke before you deposit them, or the daycare woman could have a dialogue that goes like "Oh is that a amnesia token?" >>> "Would you like to use it on this pokemon?" >>> *Y/N*. As for acquiring them, i thought they could be picked up as a random item outside of battle anywhere (not dependant on location). And they could be tradeable to create new ways of making money on the GTL. As for the drop rate, i think they should be pretty rare to come across.