"When I was programming on a daily basis, I did make code for testability purposes but I hardly did write any unit tests. However I was renowned for my code quality and my nearly bug free software. I like to investigate WHY did this work for me?"

Here's why: anything that focuses attention during development improves quality. Unit tests can be that thing but that means that their value is in a place that Cope is not describing.

See: http://michaelfeathers.typepad.com/michael_feathers_blog/200...