Why was diary-fancy-display-mode conditionalized?

[Stephen Berman <stephen.berman@gmx.net>]

As a result of this change:

2009-09-04  Glenn Morris  <rgm@gnu.org>
        [...]
	* calendar/diary-lib.el (diary-fancy-display): Only switch modes if
	necessary.

diary-fancy-display-mode-hook is only run when there is no live buffer
whose major mode is diary-fancy-display-mode; i.e., in effect, when
there is no fancy diary buffer.  Previously the hook was run
unconditionally.  Since calling diary-fancy-display when there is a live
fancy diary buffer erases and rewrites the buffer, but does not change
its major mode, this change means that, in this circumstance, any
functions in diary-fancy-display-mode-hook will not be called, which
can, e.g., result in a surprising change in the appearance of the fancy
diary buffer (I have such a function in diary-fancy-display-mode-hook in
my init file and was a victim of such surprise).  This seems like a bug
to me, and I think the simplest fix is to revert the change; or does it
fix some other problem?  If so, then how can the failure to run
diary-fancy-display-mode-hook be repaired?

Steve Berman