Great question. For classes with sufficiently rich customization options (UISlider, UIStepper, UIBarButtonItem, etc) I don't think it's necessary to provide a full-on subclass, as ultimately I'm not really changing what the class does. I'm really just providing a mixin for a particular set of UI customizations (setting colors, etc), which categories are perfect for.

Some classes (UIAlertView and UISwitch, most particularly) aren't very flexible in terms of appearance customization, so I had to reimplement them to achieve desired effects. Using them is more difficult: to use an FUISwitch (my UISwitch clone) you have to replace any UISwitches in your app with FUISwitches (also, if you were using categories/subclasses of UISwitch to do some custom stuff, they would break).

Moving logic to a base class isn't really possible, as most of those classes already inherit from UIControl (and I'm not crazy/motivated enough to reimplement them all from scratch - UIKit is awesome, and I want to lean on it as much as possible).

The single header totally makes sense, I'll be sure to add that.

TL;DR: UIKit is great, I don't want to reinvent the wheel, categories are really easy to integrate into existing projects.