[morelisp]

In my experience, I have never seen people accidentally (or ignorantly) wire a stub to a stub. When you do this it is really obvious, because there are only trivial, or maybe no, assertions possible.

I have often - like on a team of 5-10 at least 2x per year - seen people wire a mock to a mock or a fake to a mock, and either not realize it (because it's under six layers of DI) or just they're not paying attention but someone said they "have to write tests" and trying to get it over with. And this is generally a lot less obvious, because the test suite is still full of assertions, sometimes even totally reasonable-looking ones. (Such tests are even sometimes "real tests" - but that the mock is doing what's expected, not of the desired SUT.)

There's also the general argument from the principle of least power. The less code in your doubles, the more certain you are the doubles are doing what you expect, and the more you're verifying what really matters in your code. Like how pure functions are preferable to impure functions when possible, having comprehensive tests without concerns about "side effects" also means your code has fewer relevant side effects.