After coming with a discussion about unit tests and layers, I got a bunch of other insights, specially from a video of Gary Bernhardt about "Fast Test, Slow Test".
Just after posting about who one could see the layers through unit testing, I finally watched a video of Gary Bernhardt (of the "DestroyAllSoftware" fame) about "Fast Test, Slow Test":
Basically, what Gary is going after is saying "write tests for the layer ONLY, so all your tests are fast". Ok, I can get behind it, in a way. Because layers have their behaviour -- in a "mechanical" sense, since they don't require human interaction, but behaviour nonetheless -- so you're testing behaviour.
But that also rises one question: What are you testing? Are you testing the component or the application? What do you deliver, anyway? Components or an application? How do you make sure you're delivering an application in the proper way?
Testing layer behaviour also has a bad side effect: If you're application doesn't need a certain part of your layer -- say, you wrote a validator in the model layer, but the powers to be decided it wasn't required anymore --, how do you make sure it will go away? Your tests will still test those validators -- after all, you're testing your model layer -- and your coverage will still point that that piece of code is needed and you'll end up with a bunch of dead code that is kept alive only because the tests require them.
Also, because Gary points out that the "integration tests" are still required, you'll end up with a lot more tests than necessary. Why not focus on the behaviour your application should have instead of the behaviour your layers have?
I'm not against layer testing per-se, I'm just against writing tests that do not reflect the general expected behaviour of the application and duplicating tests because you're testing layer after layer and then testing them all together. You should test the value of your application, not its components.
On a side note, Gary still does the same mistake everyone does, calling the "all layers" tests "system tests". That's wrong. Just because you're going through different layers it doesn't mean it can't be a unit test. It depends only on itself? Does it test behaviour? Congratulations, you have a unit test.