I believe that there is a fine line between what needs to be bundled and what doesn't.

In the case of social sharing, adding a simple set of buttons to the theme, shouldn't be a big issue and should definitely remain core to the theme. If it is going to be a lot more advanced then maybe it should go into a plugin.

A mid-way solution could possibly be to have the theme recommend and prompt a set of plugins that best work with the theme. The output of these plugins can be integrated in the theme code so that it doesn't need a user to try to style them and make them look good.