I would say that 100% coverage is useless because you're chasing the wrong metric. Covering all branches doesn't mean you're covering all use cases. We should be chasing case coverage instead. The problem is, of course, there's no way to measure that reliably and automatically.
Additionally, there are parts of many applications (especially web apps, but not exclusively) which are inherently integration points and should not be unit tested at all, so 100% coverage is actually bad there because you're unit testing integration which is not only against the idea of unit testing but requires a ton more work. Testing these pieces of code require mocks and other stand-ins which are almost never required in our unit tests.
Having said all of that, we do in fact aim for 100% coverage of the non-integration files. But again, that's only because it's measurable and case coverage really isn't.
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