Warm Standby and resetting the primary as a standby

I've been reading up on the documentation for WAL shipping and warm standby
configuration. One concern that I have (a common one, I'm sure) is that it
seems that after bringing a standby server up as primary, other standby
servers (including the original primary) need to be rebased before they can
read the new primary's WALs in continuous recovery mode.

It seems that the cause of this is a change to the leading digit of the WAL
files:

http://archives.postgresql.org/pgsql-general/2010-03/msg00985.php
http://archives.postgresql.org/pgsql-admin/2009-08/msg00179.php

I was hoping that someone would shed some light on this situation with a
technical explanation. It's not clear to me why the WAL files are
incompatible or why the digit increases. What does that first digit mean to
postgresql? Is it possible to have the restore_command ignore the leading
digit?

I expected the WAL files to be compatible. If I start two servers from the
same "disk image" and then they get the same exact changes recorded in WAL,
why should the next created WAL differ depending on which server creates
it? I imagine these two servers to have identical new versions of a "disk
image" after consuming the exact same WALs (one generated them, the other
read them).

I'm surprised that this question doesn't come up more often or that there's
no explanation in the docs about why its necessary to rebase a primary that
went down gracefully (e.g. for planned maintenance)

Thanks

Derrick