What are the benefits of Persistence Ignorance?

I am a newbie in the DDD+TDD World. But I have been in programming for almost 9 years.

Can someone please explain me the benefits of persistance ignornace ? Typical nHibernate application just pushes the dependency between class and database to mapping files.

If I change Class files or Database, I have to change the mapping files. So isn't it just pushing the dependency by adding one more abstraction layer? In my opinion so far I don't think it's anything revolutionary. But I am not sure if I am missing something.

Finally How can I test the mapping files ? There are chances bugs will appear from mapping files, how can I test them?