I cringe when I hear people talking about the need to reach 100% test coverage because the return on investment after making it to that point is pretty low. The true value in testing lies in the validation that critical code functions properly and continues to do so after features are added to the system.
An article on the BitRoar blog has a lot more to say on this subject:
I’m not saying TDD is a bad thing, but there are more tests than unit tests, and there are more ways to verify software than testing. If you choose a methodology without comparing it against alternatives, if you claim that it works all the time without evaluating the results, than you’re not doing engineering - you’re practicing a religion.
The article picks on TDD specifically but I think the premise applies to all types of testing.