I'm a died in the wool TTDer, but I actually think that mocking unnecessarily is usually harmful. I use it as a technique of last resort.

I should define my terms before I explain, because many people use the term "mock" to mean things it didn't originally mean. Test objects that are used in place of domain level objects were traditionally known as "fakes". A fake that represented a fixed known value was called a "stub". A fake that included an assertion that a function was called (or that collected data on function calling) was called a "mock". It's a bit confusing for me that many people use the term "mock" to mean "fake". I found it weird that the original article pointed to an article on faking and then used the term "mock" without referred to the original meaning of the that word (which makes me wonder what they mean when they say "fake").

Anyway, usually you want a fake when you don't have access to some part of the system to test it directly. Sometimes that's because it's a completely different service. You can fake out that service so that you can see if the code that interacts with that service is working, without having to actually set up the service.

A stub is useful in situations where you need to know that your code is working with specific values of data inputs. So you might have an object that you pass to a function and you want to know what happens if one of the properties on the object is null. It might be hard to set that up, so you can stub it out.

A mock on the other hand, basically tests if a function is called. A good example where you might legitimately need a mock is where you pass an object to a function and you are expecting that a callback on that object will be called. It's really hard to test that without a mock.

Where mocks can be dangerous is when you completely mock out any interfaces and stub the return values. You pass a fake object as a collaborator to your function and you test that your function works. The problem is that your fake object may not necessarily represent a real object in the system.

If you ever want to refactor the code, your tests will no longer tell you that a property is missing, or that a function is missing because all of your test code is using fake objects with mocked and stubbed methods. Ideally a unit test that uses an interface should fail when you change that interface. This allows you simply to change an interface somewhere and have your tests tell you exactly what you need to do to make that change work.

Where you end up getting a lot of conflict WRT testing strategy is that some people believe very strongly that unit tests should test things in isolation. Secondly people believe that unit testing should be a black box testing strategy. So you should test through your public interfaces only and any collaborators that adheres to the interface contract should work as expected.

In this style of testing, you are often encouraged to mock anything and everything at the interface boundary. This has many advantages. First, it means that your test objects can be very simple, so writing tests is very quick -- even if the code in the system is complex (because you aren't using any of that code). Second, because you are testing the public interface only and there is minimal setup, your tests become documentation of the interface contracts. Third, because it is black box, if you change the implementation of your "unit", you don't have to change your tests.

Despite these benefits, I'm not a big fan of this style. I like white box testing using real collaborators. My goal is not to define interfaces and nail them up -- quite the opposite. I want to be able to change interfaces fluidly. I value ease of refactoring over just about anything else. Second, I want to use real collaborators almost because it is painful. If your collaborator is awkward and brittle to set up in tests, it is also awkward and brittle to set up in production code. My goal is to remove that and to simply the code. Again, my highest value is my ability to refactor the code. I want the code to become easier to work with over time, not harder and more complex. Finally, I want to code to break at a "white box" level, not a "black box" level when I change behaviour. Ideally, I want my tests to say, "On the third line of that function, we're going to have a problem because that function is different now". I don't want to be aware of problems at a larger scope "Somewhere in function A that calls function B which calls function C and D there is something wrong because it does something weird".

In the end I write small functions that are tested directly with real collaborators. I avoid private functions because it hides my implementation details. I test at a low level so that I avoid test complexity from excess branching. I get incredible specificity from failing tests, when end up essentially giving me a TODO list for what I need to do when refactoring code.

Hope that gives you some idea of at least why one person avoids mocking -- although, you do need it sometimes. And to be fair, sometimes I'll do a London School, outside in, mock the world implementation if I'm not sure what I'm building. However, I throw away all my mocks and re-TDD once I know what I'm building.