Bertrand Meyer had the right of it, but I had to figure this out myself before I ever saw him quoted on the subject.

Me:

Code that makes decisions has branches. Branches require combinatoric tests.

Code with external actions requires mocks.

Therefore:

Code that makes decisions and calls external systems requires combinatorics for mocks.

Bertrand, more (too?) concisely:

Separate code that makes decisions from code that acts on them.

Follow this pattern to its logical conclusions, and most of you mocks become fixtures instead. You are passing in a blob of text as an argument instead of mocking the code that reads it from the file system. You are looking at a request body instead of mocking the PUT function in the HTTP library.

The tests of external systems are much fewer, and tend to be testing the plumbing and transportation of data. If I give you a response body do you actually propagate it to the http library? And even here, spies and stubs are simpler than full mocks.