"but I'm guessing that it was designed that way so that PCT Thru - hikers coming south to north could adjust to the heat of the desert in a slow long mellow fashion with full packs as they transitioned from the cool mountains down into the desert floor - just a guess. "
It's the engineers who did it, I think. The Forest Service built the trail to a strict 10% horse grade, a slope gentle enough so horses won't erode its tread. Much of the PCT is built this way. For hikers, this overly-gentle grade is sure frustrating! I just hated it when I descended to Snow Creek in June of last year. A 15% grade would have been fine, and would have cut its length by 33%. They could have avoided a lot of brushy slopes with a steeper grade. Up to 20% is o.k for hiker trails, though that is getting to be a pretty brisk climb (1000 feet per mile). Steeper than that and trails tend to erode like crazy.

