This is probably the first time that i'm disappointed with new features that are added to WordPress. I love WordPress and i like the theme customizer as it is, meaning it should only customize the current theme. I personally do not see any point in adding a theme switcher in there and i will probably never use it. I keep reading how theme developers should stop bloating their themes but then it's the customizer that starts getting bloated? With all due respect to the people that worked on this feature, it looks nice and fancy, but i don't see the point of it being into the core, my opinion is that it should stay as a plugin, but i guess it's too late now.