I don't have the quote on me, but a few months ago Tim Cook said that anywhere in the OS it does anything based on weather (IE, when Siri tells you the weather, or on the notification screen), it's pulled from the iOS weather app. He said there's a lot of surprising ways throughout the OS that they're dependent on those default apps.
Which, as a software engineer, sounds like it means iOS has been very poorly designed. Functions which should be defined in the core library for the OS, or within shared libraries (a notion which iOS doesn't support) are instead defined within apps. Which means the OS cannot function without the presence of those apps.
Although really, it seems like Apple could easily band-aid over this by just removing the icon from the homescreen when you "delete" it, and then allowing them to be "installed" via the App Store app.
I don't think it is hard to move those functions to some system level daemon.