Hi Phil
If I knew how to draw diagrams with photoshop, I could post some on here to make it more clear.
What the engineering types say is that a floor that is a lot longer than it is wide, will experience more stresses (and uneven stresses) than a floor that is roughly square in the layout.
If a room is for example 8 feet X 24 feet, then for best practice the length of 24 feet should be divided into thirds with soft joints. This would then make three separate tile installations of 8 feet X 8 feet. As the tile experiences stresses from heat/cold, sunlight, floor loading and deflection, etc, the three separate 8' X 8' fields of tile will perform better than one field of tile 8' X 24'.
Most customers don't want to see anything going through the field of tile, not a soft joint material or Schluter Dilex or anything else. It's an uphill struggle trying to get folks to agree to this, but the facts are the facts and these are the recommendations from the engineers.
I try to get a compromise and get one in place, whenever possible. And I try to place them where they blend in, like in a doorway, or going under a island cabinet, or in some way that is less noticeable.
I have a kitchen/breakfast room floor coming up, and I'll place a soft joint at the end of the run of cabinets going to the back door. It will only be 4' long, but will nicely divide the two rooms without anything being too noticeable. It won't get the ideal ratio of 1.0 to 1.0 for each section of the floor, but will end up with the kitchen at about 1.5/1.0 and the breakfast room at 1.0/1.0.
I've never gotten a clear answer from Schluter about this, but I think for a warranty claim for Ditra, this is one thing they look for when evaluating a potential floor failure. I've had discussions with my local reps about this, but haven't taken the time to see what is said in writing in any warranty information.