Some people like oranges, some people like apples. (har har). My reason for changing UI is the same reason I don't use certain applications. Other applications, whether for purchase or open source, have features and functions that the base OS doesn't often provide.

You don't type reports and documents in notepad even though it is a serviceable program. You get Open Office, MS Office, or some other word processor. You can listen to music using windows media player, but there are third-party programs which are far superior to that (and iTunes, IMO).

The iOS is fine and polished, but it can't do certain things that I would expect it to do-- like widgets for instance. Sometimes I'd rather just glance at my phone and read the subject of new emails or texts without having to open the program. Being allowed to customize my phone allows me to put my most commonly used applications on a home screen and relegate the rest to the application tray-- just as I would on my home computer. Little things like that are nice.

Henry Ford revolutionized car purchasing for the average family. Apple is revolutionizing smartphones for the average consumer. I think that android has a nitch home with techies. The openness of the OS is allowing that same base OS to potentially become a solid alternative choice to iOS for the non-techie consumer. The flexibility of android may be what puts it over the top. Much like windows provides a flexible experience, it's wrought with more possibilities for user error and catastrophe. Apple provides a predictable and controlled experience that rarely gives a consumer consternation-- but at the expense of flexibility.