Imagine someone implements an awesome new feature for dired. Emacs
users the world over are amazed by this, and fill their blogs, twitter,
etc. with the news. If dired is an ELPA package, everyone who hears
this news can get the new feature in their emacs instantly by upgrading
their ELPA packages. No need to wait N months for a new release of
emacs, or compile a non-release version of emacs from git.
How is this different when Dired is in the Emacs repository? The
Emacs repository is a public one, so anyone and everyone can get the
latest version from there and use it, if they want.
The suggestion was to move _all_ of them, except the few that are
needed for bootstrap, out of the Emacs repository. Most of the
packages in that category are neither like Org nor like kermit. They
are relatively small, but get quite a significant number of changes.