"All calls can have side effects" - this isn't the case for a "pure" function. What the author wants would make some code worse but is very useful for anything

"All calls can have side effects" - this isn't the case for a "pure" function. What the author wants would make some code worse but is very useful for anything working with immutable collections [IMHO, reading #4 that's what's behind his complaints - working in a functional/immutable style where C# isn't entirely comfortable]. There could be a middle ground where certain function calls do warn/error on such use and most others do not.