How cross-functional should my Team be?

In a Lean/Agile world teams should be cross-functional. Period.
Or maybe it’s not that easy? What does cross-functionality actually mean? And is it true that the more cross-functional, the better?

I wrote an article together with my brother Arne Roock adressing these questions. The article is published in the current edition of the „Agile Review“ by it-agile. The article is available as PDF for download. (There is also a german version of the article).