I had the exact same thought about having rarely seen the center post supporting a ridge beam. Why not? (Although, Ted Benson shows this design in his 1997 book, The Timber-Frame home: design, construction finishing)
One possible explanation for the rarity of this design is that the savings in wood that you would get by having one full length beam (the ridge beam) over two mid-span plates is negated by the need for larger, full length rafters.
Since I already have the rafters in hand, I cannot see any reason why I wouldn’t take advantage of the ability to transfer the roof load directly to ground through the posts. This eliminates the need for over sizing the tie beams since they will be relegated to the principle task of supporting second story floor load.
-Dave
