Presumably WP.com are introducing this due to demand from users to modify those elements.

If there's demand for it among WordPress users, wouldn't it make sense to include the functionality in core itself so that theme authors and users could take advantage of it without having to use Jetpack on self hosted installs?

Authors could support it through add_theme_support()

If you're a theme author who receives support tickets from users about how to modify the byline info or excerpts, telling them to install Jetpack just to modify it is overkill.

A standardized method of dealing with it in core would seem to be in everyones interest, like the custom logo functionality.