Agreed. 100% line coverage doesn't mean that all functionality is tested; You don't test the full range of values every variable can take on, all possible throw-catch pairs etc. So the question is if the difference between 80% and 100% line coverage is that big. In fact, I think it can be decieveing to use the term '100%'. Arguably the reability of your program can be increased from 100% line coverage by adding an extra non-tested value check, effectively lowering the coverage.
