window configurations no longer (since 21.5) include windows ! ?

[Stephen J. Turnbull]

Henry S. Thompson writes:

 > one substantial application, namely ecb, depends on identity
 > preservation.  The substantive question is: what's the real
 > functionality that is lost when identity preservation is lost, and
 > can we get it back w/o losing the major benefits of the 21.5
 > promotion of much of the window code from C to elisp?

How does ECB use window identities?  What is the "real functionality"?
("In 25 words or less" :-), I'm looking for a design pattern, not a
detailed internals document.)