The less you allow a consmer to customize a product the stronger the brand recognition remains.
The iPhone's branding visuals are:
1. The apple logo on the back of every iPhone
2. The familiar square icons lined up in a grid
3. the familiar circle home button on the bottom.
I jailbreaked my iphone, put it in a case, used a Winterboard theme to change the way all the icons looked and you had to see the confusing looks I got from people when they looked at my phone.
"Is that an iPhone?".
You'd be surprised how these subtle changes confuse people.
If you notice the only UI customizations in iOS 7 relate to accessibility. Of course wallpaper being there from the start. I really don't see Apple allowing users to customize more and more aspects of the UI.