mediawiki-l@, please
OK, sorry. Next time...
I don't know why the design decision was made to just leave those Mediawiki: namespace items sitting in the archive and text tables. But
Just run update.php. It does all that crap for you.
I am sorry but update.php just calls deleteDefaultMessages.php which just leaves them sitting in the text and archive tables forever!
It just does a courteous delete, leaving all 1500 yes, deleted, but not expunged, forever.
If your wiki has been around since early 2007, have a look in your archive and text database tables. The 1500 hundred message strings should be very obvious there in the text table, and their corresponding items in the logging and archive tables.
Years and years will go by and one day you get curious and look in the database and surprise, for a small wiki perhaps 90% of the database is occupied by useless bloat.