There are two problems with that:
1: The message never pops up until _after_ you've unplugged the charger, thus it's useless.
2: The charger doesn't actually suck any power from the outlet when it's finished charging. There's no drip-charging of the battery, and the switch-mode charger is so efficient that what it uses when not charging is neglible. A user here on this board used some sophisticated equipment to measure the standby current on the N8x0 chargers, and came up with a wattage so low that the energy spent on the pop-up message (turning on the display and all) is much more than what you "save" by disconnecting it. (Obviously on the older devices the message actually popped up when the charging was finished, not when you unplugged..)