So you can avoid security problems without complete control, but you also mentioned the UI, and it is just one part of the overall experience. E.g., today most

So you can avoid security problems without complete control, but you also mentioned the UI, and it is just one part of the overall experience. E.g., today most people don't really differentiate parts of their experience defined by the OS from those defined by application makers. So you have to control the ways your users install software, if you want users to not hate your product because of some crappy app. To be honest, I doubt that without such complete control there would exist a computer / tablet I could recommend to, say, my non-technical mother, and be more or less sure that she won't hate all computer-related things afterwards.