After some additional Twitter discussion with Jimmy last night, I realized that my previous response had neglected to distinguish between two very important contexts - new code and legacy code.

Considering a legacy system (as defined by Michael Feathers), I'm 100% on board with everything Jimmy is talking about and all of the techniques, troubles, and diminishing returns that he identifies.

I still maintain that 100% coverage should be the default result of true TDD when writing new code - and I apply this philosophy when writing new code inside of a legacy system. Write a test to prove you need the new code that you are writing... unfortunately, the distinction between new and legacy code does get blurry and can be difficult to navigate when working in a legacy system.

For more info on legacy code, in this context, and how to deal with it:

• Working Effectively With Legacy Code - the book by Michael Feathers
• Working Effectively With Legacy Code - Article on ObjectMentor by Michael Feathers (consider it the cheat-sheet for the book)
• Jimmy Bogard - He's done a "Legacy Code" presentation several times, based on his own use of the Michael Feathers techniques.