>There's an argument to be made that to the extent they break the rules in some manner, they're doing more harm than good. Should you be testing scenarios that are impossible to encounter with the code?

Yes actually, especially when it comes to i/o. Let's say I want to simulate how many code handles a REST interaction with a third party service. I want to be able to mock the REST callout without performing any actual i/o. This is why these test frameworks are powerful and useful.

Later if I want I can test to see if the actual callout works by hitting a local endpoint.