Having 100% code coverage is like having 0 warnings (although it certainly is a lot harder). In this situation, your tools are not telling you "all's good", but

Having 100% code coverage is like having 0 warnings (although it certainly is a lot harder). In this situation, your tools are not telling you "all's good", but rather "I can't detect anything suspect here". There's a good chance that the dev time needed to go from 90% coverage to 100% coverage might be better spent somewhere else.