Given that there appears to be no hard and fast rule as to what a good level of code coverage for unit tests is, what then are the benefits of using a code coverage tool such as NCover?
It is fallacious to measure software quality solely according to a code coverage percentage, as you've pointed out. But NCover allows you to examine exactly which parts of your codebase have been neglected by unit-testing. Since you should know which parts of the code are executed most frequently and which parts are most error-prone, NCover is useful for ensuring they're at least being tested.