plastic behind cement board with waterproofing is essentially suspenders when you already have a belt. the waterproofing is also a vapor retarder so if the red guard or kerdi product is continuous, lapping corners and all, the plastic will be unnecessary.
if you think of tile like exterior cladding, it would make sense to leave an open hole in the sealant in line with a vertical joint in the grout. porcelain and ceramic tile is impervious (some soft specialty limestone or sandstones are not), so any water that migrates does so in the grout joints (which crack on occasion as you know). it should then hit the waterproofing and migrate down (however slowly) to the "weep" in the tile joint around the lip of the tub/shower. ideally, you would install the vertical tile over vertically troweled mortar, which effectively provides little paths for the water to drain within the wall. EIFS manufacturers have tried and tested this strategy such that it is their only warranted water management technique.
this is why you eventually see black mildew stains in the grout around the base of the tile - because the water is stuck back there. the redguard product is a vapor retarder which means the steam (water vapor) from the shower won't migrate thru the substrate, it will hit the waterproofing and, assuming it condenses on the colder surface migrate down along the same path noted above. in most instances, the tile is colder than the cement board, so the vapor condenses on it first and stays on the exposed surface. rather than continuous plastic sheet.
as compromise, I would suggest a 6" high strip stapled above the flange and lapped over the flange and under the edge of the cement board is an adequate solution to prevent overflow situations. probably unnecessary on a tub, but possibly on a shower pan. then waterproof the walls, install the mortar with vertical troweled grooves, set the tile, wait, install sealant under each tile, stopping at each vertical joint for a small gap, plug the gap with a spacer before grouting, then fully grout, remove the spacers, and clean the tile. the holes will look a bit odd at first, but will serve their purpose.
this comes from me, an architect, who spends a lot of time thinking about how to keep water out of a building. I was told once "think like water" and you'll figure out where to let it out.
no flange provides a different challenge. i'm about to rebuild my own shower with a tile floor and am thinking thru these exact details before buying anything. thanks for talking about it and making me think like water for an interior application.
