This whole post applies to desktop browsers and cookies too, but the use case is underscored on a mobile device because user input is relatively more difficult.
As a mobile app user (e.g. iOS apps) I want to tell an app as little about myself and my preferences/needs as possible in order to get to the functionality I need quickly, because authentication steps and preference settings are excruciatingly painful to use, no matter how efficient they are.
The authentication step, on iOS, is getting better thanks to Twitter being baked into iOS. However, I've found that many applications drop my auth/pref state after I do an App Store update of the app. That has resulted in me treating updates as radioactive, not because they're likely buggy, but instead because they'll likely blow away my auth and/or app-specific state (e.g. previously selected items or favorites within the app), and going through the auth steps (entering a username and password, or clicking through oAuth steps) is just too much of a PITA.
I'd ask that app stores require the developer to disclose whether or not the update will break user-specific state, so I can decide whether or not to apply the update. Without this, I'm finding that I just don't install updates anymore out of fear that they'll break my state.
I'd also ask that all of this state/db be backed up in the cloud somewhere so when device firmware resets are done, all that app-specific user state can be restored.