[BJanecke]

This is fairly dangerous advice and amounts to. "Hey folks, I think it's a great idea to friction weld parts of your code together"

It's not YAGNI or duplication, if you can't understand why we program to an interface and not an implementation then you just very seriously need to go look at your codebase and decide to extend a core bit of functionality, one that usually comes to mind is Authentication/Entitlement