Been discussed before to much disagreement. Personally I would rather have the carriers test updates before being pushed down to users. I do not want to end up with a bricked phone because such and such update didn't get tested well enough.
Of course the obvious counter arguement here is to put that responsibility on the OS company. I still prefer the current process of the OS provider AND the carrier testing before release.
I prefer the Apple model. Keep the carriers out of the process. The lack of carrier testing isn't a problem in that eco-system, so I don't see why it should be in the Windows Phone eco-system. It seems Microsoft went with a system that falls half way between the anarchy of the Android world and the Walled Garden of the Apple world, and I'm not sure it's working too well.