Steve Voigt
John and Wiley,
I understand what you're saying, but in practice, levering the chisel to clear chips will accomplish the same thing. I know if I'm cutting a furniture mortise, every speck of mortise wall gets scraped more than once during the process.
For me, the main reason to keep the chisel vertical is that it makes it easier to avoid tilting the chisel right or left and screwing up the width. Again, that's for a furniture mortise.
One advantage of Follansbee's approach is that you can deliver more power when the mallet strike is in line with the direction the chisel moves. So this is my approach when the walls are going to be pared and perpendicularity is not as essential…for example, in planemaking.
I started out mortising bench planes with a chisel, bevel down (like I'd do a furniture mortise). I later switched to roughing out with an outcannel gouge; that was a modest improvement. Finally I switched to an incannel gouge, used the way Follansbee uses a chisel, and that was better still. It's just faster. So, different approaches suit different situations.