What makes something not necessary? The fact that N_LaRUE doesn't feel that need?
A feature is unnecessary if:
- it doesn't solve an actual problem
- it introduces more problems than it solves
- it doesn't solve the problem as good as an alternative solution
The desire to replace built-in apps isn't really unnecessary, but it is at least nonsensical, because it's not possible to remove apps from ROM. What Android can do is hide the app's icon, so it doesn't appear in the app launcher, but the app is obviously still there. WP's equivalent is simple... don't pin the app to the start screen. That is probably the superior solution, so I'd agree with N_LaRue that the proposal is unnecessary.
The desire to install non-store apps is definitely a solution that introduces more problems than it solves, so it's worse then unnecessary. It's counterproductive. I don't know anything about the non-store XAP that WPC linked to, but whatever that was about, I'm sure a better solution exists.
Not every feature that can be implemented should be implemented.
The 'built in' apps are not apps, they're part of the OS.
This depends on your definition of the term "app". I think the built-in apps are apps, almost just like any other. For example, WP 8.1 will (apparently) ship without the Xbox Music and Video apps, delivering them through the app store instead. IMHO this clearly demonstrates that neither are necessary for the OS to function, meaning they aren't part of the OS... they are just apps that
are bundled with the WP 8.0 OS. Most user facing software is similar. You could remove most of the apps without negatively impacting the OS' ability to function correctly.
Applications like MS Paint or Movie Maker aren't part of the Windows OS either. They're just software products that MS bundles with the OS.