We used Ditra under the stone tile in our kitchen. It worked exactly as advertised, easy to install, etc. But you have to read the literature carefully to fully understand what its purpose really is.
Whether you use backer board or Ditra, the assumption is that the subfloor is sufficiently strong and rigid to support the tile without flexing so much that the tile cracks. Neither Ditra nor backerboard are designed to add any strength to the subfloor. That is why most of the tile manufacturers recommend that for 16 inch on center joists, you have two layers of subfloor, typically 3/4 inch plywood or OSB, covered with either 5/8 inch or 3/4 inch plywood or OSB. The combination is very rigid, and very strong, and needs no additional strength.
So, assuming you have adequate strength, the purpose of the Ditra is to provide an uncoupling mechanism between the tile and the subfloor. Effectively, the tile is rigidly bonded to the Ditra via thinset (that is what all the depressions in the Ditra is for), but the Ditra is capable of moving independently of the subfloor. If your tiles are rigidly bonded to the subfloor, you can get cracking of the tile if there is differential movement of the subfloor and tile. In climates where the humidity changes significantly over the course of a year, you are going to get substantial movement of the subfloor, with little movement of the tile, so with no uncoupling membrane you risk stress buildup between the wooden subfloor and the tile. Kind of like earthquakes, the stress can get relieved via a crack. Bad news. With the Ditra, the subfloor moves, but the movement does not translate into the tile, and you avoid cracking.
The fact that Ditra is waterproof is a bonus, not its main purpose. Ancient tile technique used a layer of loose sand under the tile as a bond breaker, very effective. Use of relatively weak mortar (full mud set) can also be effective in minimizing cracking, since the mortar can take up most of the subfloor movement. But with the advent of modern, high strength thinset, lack of a bondbreaker invites cracking of the rigid tile. So that is why contractors like Ditra, it minimizes the potential for cracking. But homeowners may not understand what its real purpose is, and may balk at adding $2 per square foot to the job.