[vsareto]

They're not required for MS's dependency injection, not sure about other libraries. 95% of the interfaces I've ever seen used for DI only have one implementation, and that 5% is generous. This makes maintaining the class and the interface tedious.

The reason this is so common is probably because code examples (including MS) often have it, so it gets followed the first time and repeated.

[throw1234651234]

Absolutely my experience is well, I have been writing .NET for over a decade, and I think I have seen a handful of cases mainly when we were bored or when I remembered the Interface Segregation principle and tried to shoehorn it into code (though the principle does work well to have methods take the minimum number of props needed to do their job, but that's a whole other topic).