I think this is the crux of why tests are useful. You can't write good tests without thoroughly understanding the code under test--and its dependencies. And far too many developers only have a hazy idea how their own code works, let alone the code they're interfacing with.

When developers have to write good tests (and code review can help here), they magically transform from morons into not-morons, without anyone poking and prodding them with best practices. The tests themselves are secondary.