it might be more useful to concentrate on comparing P5 to P6, showing equivalent code in both versions, with new, P6-specific idioms.

It might, but that's not the book we decided to write. Our target audience are not only Perl 5 programmers, but more generally programmers interested in Perl 6.

On #perl6 people ask about introduction to Perl 6, and they have no prior experience with Perl 5. Sending them off to learn Perl 5 first to then forget a third of it and learn something else -- that feels just wrong.

But still there is more Perl 6 literature for the Perl 5 savvy programmer (like my blog series, and various other documents).