I have a Manitou Black with reverse arch (an old model) and it seems to me that the posts get dirtier than if it was a front arch model. Specifically, the grime that comes "in the air" hits the posts directly (without the arch acting as a sort of protection), and the mud that comes with the tire ends up getting everything dirty anyway, sooner or later. Also, since the O-ring around the posts is tilted backwards due to steering angle, mud actually tends to form a deposit behind the post.
Incidentally, I have rim brakes attached to the brake posts, and due to the arch being behind, I am forced to use a brake booster, otherwise the blades of the fork flex A LOT while braking, with poor performance.
In the end, I agree with the ones that think it's more marketing than engineering. Besides, if you Google "dual arch suspension fork" there are already some models around, and I have seen at least one "middle arch" fork, though I can't remember the brand.