Jason Gorman ruminates on unit testing on his excellent blog. He makes a strong case for doing all the testing categories. Unit and component tests are important.
Unit tests can’t prove your product works, but they certainly can prove that it doesn’t. When they do you know to the line of code where it went wrong. That’s pretty cool if you ask me.
To extend his automotive metaphor knowing that spark plug #3 isn’t sparking is a lot more useful than “The car won’t start.” If you skimp on unit testing, you will find yourself starting product diagnoses from the “car won’t start.” position all too often.
Use cases (scenarios) are the most important tests you can run. However, you can’t run them effectively if you don’t build them on a solid base. Abraham Maslow posited a pyramid model of human needs where the low level needs must be met before the high level needs can be fully realized. Testing is just the same.