pl/pgsql enabled by default

Is there a good reason that pl/pgsql is not installed in databases by
default?

I think it should be. pl/pgsql is widely used, and having it installed
by default would be one less hurdle for newbies to overcome when
learning PostgreSQL. It would also make it easier to distribute
applications that depend on PostgreSQL and use PL/PgSQL: rather than
saying "You need PostgreSQL, and then you need to do [ createlang stuff
]", those applications can just depend on a sufficiently recent version
of PostgreSQL.

AFAICS, the overhead of installing it by default would not be large:
just an extra row in pg_language and a few rows in pg_proc. So I can't
really see a major reason *not* to do this -- am I missing one?

A related issue is where the PL validator and handler functions for
PL/PgSQL would be placed if it was installed by default. I think placing
them in pg_catalog (rather than public, where they are currently
installed by createlang) would probably be best.

-Neil