In reply to Rich Tabor (ThemeBeans).

No, the purpose of the Customizer is to Customize your site; ideally the customizer has all options related to the front-end of the site regardless of whether they're added by themes or plugins. Adding theme-switching to the Customizer clarifies the difference between themes and theme options, and makes the live-previewing process more centralized to the front-end context instead of the admin.