Why just unit tests? Integration tests seem much more valuable as documentation of what the users will do in the app. Unit tests have limited benefits overall, and add a bunch of support time, slowing down development. If you have good (90%+) coverage just from integration tests, you are likely doing 90%+ coverage of the unit tests at the same time, without the extra effort or support burden. You can use the same reasoning to describe the benefits for understanding the code, you get a clear understanding of the important usage cases, plus you get the unit-level "documentation" for free.
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