The example I provided is contrived. Where I’ve used it before is to check that an object passed as a constructor argument isn’t null. I don’t mind using ==null instead of is null but the VC# compiler accepts both. I think the edge case that the is operator avoids issues if you’ve also overridden the == method for a class.
It has some initially odd-seeming side-effects, but far from crazy imho. And ironically this seems to fit with C# increasingly embracing English-like syntax, more commonly associated with … Pascal (see also: not, and + or in C# 9).
I guess they’ve finally run out of hieroglyphs to construct out of the standard symbols on a Roman character-set keyboard.