This sounds like nonsense to me, but I will give you the benefit of the doubt and ask you for *concrete* examples of what you are talking about. I have yet to be cited a single good example here - very often what is being done would work just fine in HTML, with less overhead, but the 'designers' just do not understand HTML, or have any desire to learn it, so they do things this way instead.
"Is it so difficult to grok why you might want content to change on the client?"
Not difficult to understand why it was desired.
The point is it's harmful and been proven harmful, and far too harmful for the small advantages it brings to outweigh that.