contrib catalogs

This might seem like a silly question, but what are the implications of
PostgreSQL allowing developers to create custom catalogs?

For example, PostgreSQL currently uses the pg_catalog schema to store
system catalogs / relations / functions / etc.  Has thought gone into
extending the scope to allow contrib modules to create their own
catalogs (ie. a PostGIS catalog, cube, tsearch2, etc.)?

The largest advantage I see is significantly easier upgrades.  Having
contrib tables/functions flagged as a catalog, pg_dump won't include
them in a dump - restoring into an upgraded database becomes painless.

Cheers,
Kevin