If there's a requirement to fail an error a certain way, then by all means, write the test and generate the error -- but then one is generating the error to show that the requirement is satisfied, not to meet a coverage goal for its own sake.
Lacking a requirement to fail a certain way, a lot of people, myself among them, will often toss in an or die and be done with it, without ever testing that failure case to see how it behaves functionally. And, for many customers, "fail gracefully" is an implicit requirement. Coverage analysis points out where we've taken half-steps, and suggests where a few more unit (or functional) tests might be needed.
It's not about getting to 100%, though that does become tempting when being handed a color-coded chart. It's about adequate test coverage.
