Doug was referring to an article that I wrote back in 2001. The quote was concerned with a tendency in XP culture (at the time) to rebut criticism of the process by saying "Have you even tried it?" A bit like saying "How can you possibly say that jumping off a cliff is bad if you haven't tried it?"

For the record, it was a couple of years after the article appeared when we wrote the book. Like most developers, though, I've pair programmed on & off throughout my career (both in agile and not-so-agile projects), as the occasion calls for it.