A properly built shower isn't damaged by moisture even before the tile is installed. Except maybe in a commercial shower or steamroom, the shower is not typically used more than an hour or two each day, so it has time to dry out. Sealer is primarily designed to help prevent staining, but it does help slow, but not stop, moisture. On a conventional shower pan build, they put weep holes underneath the tile for a reason...gravity helps move any moisture that does get there down the sloped liner and out the drain. ON a wall, it primarily dries to the outside.
The reason I prefer a surface applied membrane is that there is so much less in a shower that can get wet in the first place on both the walls and the pan, so it will dry out much faster than a conventional shower built over materials that can absorb moisture, and, I feel, is a much better solution to a shower that gets heavy use.