I can't emphasize enough how important it is for the OS vendor to "set a good example" when designing their own apps for the OS.
Like it or not, devs will look to the built-in apps for guidance on what to make theirs look like -- this is also true for how buggy or unresponsive an app can get before it is deemed "unacceptable".
Put another way, think of first-party apps as a form of enforced competition. If do don't do better than the level we, the OS vendor, are competing on, then users will be disappointed in you.
Like it or not, devs will look to the built-in apps for guidance on what to make theirs look like -- this is also true for how buggy or unresponsive an app can get before it is deemed "unacceptable".
Put another way, think of first-party apps as a form of enforced competition. If do don't do better than the level we, the OS vendor, are competing on, then users will be disappointed in you.