What is the purpose of _NET_WM_USER_TIME?

The wm-spec does not give a very good rationale for the USER_TIME
property. It is supposed to be for prioritizing newly mapped windows,
but that doesn't justify updating the timestamp on every input event.
Some of the overhead is avoided with _NET_WM_USER_TIME_WINDOW, but it
still means sending an XChangeProperty for every key-stroke. Does
updating the time for every event really server a useful purpose?

Joe Krahn