With all of the advances in the last few years with adhesives, why would you ever consider using thinset? After all, thinset is a material that has been around for many years, is cement-based, and can be difficult for the new tiler to work with.
Thinset's "Sag" ProperiesTypically, you have a choice of two types of tile adhesives: mastics and thinset. Mastics are easier to work with and set faster than thinset. One of the great downfalls of thinsets, at least for the amateur tile setter, is that it is slow to set, during which time the tile can move out of place. The problem is compounded when working with vertical spaces such as bathroom walls and kitchen tile backslashes.
It's that dreaded "sag" that new tile-setters experience after laying tile on a vertical surface with thinset. The tile looks perfect at first; but an hour later the tiler comes back to find their beautiful tile sliding downward like houses in a mudslide.
Mastics vs. Water? Water Will Win.Mastics, though, do not hold up well against prolonged exposure to water. Tile is still considered a ready and willing victim to any kind of prolonged contact to water. This is because even the smallest crack in tile or grout can lead to infiltration of water, which can work its way around to the back of the tile, leading to mildew and mold and eventual decomposition of the backerboard.
Mastics Good For Backsplashes, WallsBut mastics can be used in areas of limited moisture. While they do not work for shower pans, bath tub surrounds, and similar areas, mastics can be used--and work well--on places like kitchen backslashes, walls, wainscots, and other vertical areas.
Thinset is ideal for those highly-moist areas mentioned earlier. As a cement-type material, thinset is inorganic and, should it get wet, will dry out quickly.
Finally it is important to note that thinset is a much stronger material than mastic, making it a better choice for horizontal applications such as flooring that get a lot of pounding.