Tom
The text below is from the Daltile website. It seems to contradict your advice or am I misunderstanding something.
Check to make sure that the cement backer board is sound after removing the old tile. If not, you'll need to remove the old and install a new backer board.
Apply a waterproof membrane to the backer board to ensure that water can’t seep through and cause damage.
The building needs the ability to "dry", either internally, or externally. Based on how I understand it, the direction is climate dependent. Buildings in "heating day" majority climates dry from one side, and buildings in "cooling day" majority climates dry from the other.
If you have a moisture retarder on both sides, any water that gets under the first water barrier becomes trapped by the second, with no escape. The membrane over the top of the substrate is different than the membrane behind the substrate. Tom's statement is accurate.
