Hmm. That's news to me, though it is an obviously common misperception. The fact is, if it is properly constructed, your shower shouldn't leak even if there is no grout or caulk (though a lack of caulking or sealant could cause extensive water damage to the wall).
Going online, I entered the keyword phrase "water damage showers" in a search engine. In the first 20 hits I found 14 such references, many from similarly reputable companies. While the misconceptions are many, it is clear that water and vapor problems cause billions of dollars in damages every year, not to mention the latest star in the spotlight, mold damage.
Anyway you look at it, improper construction and poor specification in the area of waterproofing is an enormous liability issue, and showers lead the pack by far. Someone recently asked me how many shower stalls did I feel were properly constructed. Though I have no basis for my conclusion other than personal experience, I would say maybe 20 percent will actually perform for the life of the structure.
The most common cause of traditional shower stall failure is incorrect waterproofing of the curb. The waterproofing layer under the mortar needs to extend over the top and down the face of the curb. It also should return up the jamb a minimum of 3 inches. This can be a challenge if you field-fabricate your corners, but it can be done. All sheet-membrane manufacturers make corners, and life is made much simpler when you use them. Remember, they need to be sealed, not just lapped over.
Anytime you waterproof a surface, the water has to go somewhere. In the case of the shower stall, and in many other types of waterproof installations, the floor under the membrane must be pitched to allow for shedding or draining of water. For mortar showers, the additional step of making provisions for the bed to drain around the weep holes is commonly omitted. If this is not done, in time the soap and other little goodies sitting below the drain flange in the mortar bed may cause a very moldy condition that can only be remedied by replacement and, in some states, abatement.
For surface or substrate waterproofing in direct-bond applications, the floor substrate needs to be pitched under the membrane to allow for drainage. Mortar beds should be of equal thickness over a pre-pitched substrate. Keep in mind that water sitting on a waterproof floor for extended periods may cause the floor to effloresce. It will also contribute to the natural growth of the installation, causing increases in dimension in years instead of decades.
In the age of a graying population, we are seeing more handicapped showers being installed. Many times, funds are available to assist with the refitting of these homes. These agencies pay only once, and the end-users are most often those in no position to afford costly repairs. If you are involved in a handicapped remodel, waterproof the whole floor and, depending on layout, a good portion of the walls. To stop at the edge of the shower curb will not work when there is no shower "dam" to hold the water in. The water will continue to migrate out in the adjoining areas.
While not technically waterproofing, vapor membranes also play a part in preventing moisture damage. Cool, moist surfaces can transmit water or water vapor to the inside of a wall cavity. Nearly all tile manufacturers, as well as the Tile Council of America wall methods, call for a vapor membrane. The exceptions are gypsum panels and extruded foam board. They typically don't specify barrier, which is a waterproof material with a perm rating less than 1 (4 mil plastic sheeting qualifies).
There are specific instructions in GA 216, the gypsum industry "handbook." I have never in my lifetime seen them followed, nor have other committee members, so the method was removed from the TCA handbook. If they were followed, there would be a very reasonable chance of long-term success. If a regular gypsum product is set on the tub, or the joint at the tub interface is grouted instead of caulked, the clock starts ticking. However, they say, as all backerboard and gypsum-board manufacturers caution, to caulk or seal, not grout, the joint between the panel and the tub to prevent wicking.
Whenever a panel is set directly on a tub or shower floor, or you grout a joint at the tub or base of the shower, the water will migrate up the wall substrate, possibly causing a mold issue and, in some cases, deterioration of the wall panel. If you use mastic, it may re-emulsify. In the case of shower installations, the vapor membrane behind all backerboards except those previously mentioned should overlap the waterproof membrane by at least 2 inches.
Waterproofing presents many opportunities for retailers and installers, and protection for the homeowner. This is not a hard sell to anyone who has ever experienced water or vapor issues. Properly installed, the product can provide protection against damage to substrates and protect the structure from moisture intrusion. Improper installation, or lack of either waterproofing or vapor membranes where appropriate or recommended by the manufacturer, may not only result in loss to your customer but also possibly enjoin you in what lawyers affectionately call the "Mold is Gold" group, and that is somewhere you don't want to be.