Test coverage is a useful *heuristic* for test effectiveness. Like all heuristics if you push it too far it falls apart. Get as close to 100% as is useful and don't worry about the rest.
Indeed. Brian Marick wrote a great paper on this topic - "How to Misuse Code Coverage", http://testing.com/writings/coverage.pdf.
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Danny R. Faught
Tejas Software Consulting
http://tejasconsulting.com/