Let's assume that there is a downward force applied to the beam on the left in the illustration. That would cause the other beam to bend down also and would put compression on the brace. Now the brace might be, and in all likelihood is able to support that load even though its real design purpose is to stiffen the frame. So the question is why wouldn't you want to put the scarf joint above the post. That way both beams end up in the same position and the braces can also add additional support to both beams equally well. In addition to that you would be able to put a through tenon on the post which can secure the scarf as well as held the beams in place/down(pegs).
The right beam in the drawing would be the stronger one since the one on the left is scarfed onto it and only supported by the blade and the brace but not the post.
Try to imagine the same drawing without the braces. Now where would you put the scarf joint?
Not sure if I am bringing my point across!?
