to_date() marked stable?

I was wondering why we mark to_date() as stable and not immutable?  It
seems to have been set to stable in 2006 with this patch:
http://archives.postgresql.org/pgsql-committers/2006-11/msg00264.php
Also, mark to_date() and to_char(interval) as stable; although theseappear not to depend on any GUC variables as of CVS
HEAD,that seems aproperty unlikely to survive future improvements.  It seems best to markall the formatting functions
stableand be done with it.
 

Are there people using to_date in indexes or partition functions where
changing it to immutable would be useful?  The code seems complete
enough now that we should consider an optimization here.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +