There are both pros and cons to using tile trim as part of a tiling design. Besides the potential for an aesthetically pleasing design which does or doesn’t include tile trim, there are plenty of practical purposes for the oft overlooked part of a tile design.
Tile trim has been around for hundreds if not thousands of years. It can be used as a break for the eyes from a pattern in a design, so in that instance it doesn’t serve much of a practical purpose. For the practical purpose they serve, they can provide important protection for edges and corners, protecting from damage, like chips in the tiles or just steady erosion due to heavy traffic.
There is only really one reason why you shouldn’t use tile trim, which is when the risk to the tiles is relatively low, such as a bathroom wall. Instances where they are a necessity would include stairs and floors, as there is a lot more risk associated with that.