In reply to Matt.

Themes and the ability to change them are one of the most fundamental features of WordPress. How do you feel that introducing a streamlined workflow for changing and previewing themes introduces bloat? There are no new features here, just a simplified way for using an existing, critical feature.

The Customizer exists to provide an API so that themes and plugins can add whatever features they want in a standardized way, providing a consistent experience for users across the board. It's up to themes and plugins to actually add options to it to determine exactly what it will look like, but as a core API you'll find that it's actually one of the better areas in terms of extensibility.