As for "Why bother" I wasn't looking for a smart ass answer. It was a simple question if anyone had used one and their experiences with it.
But to answer that question. It would eliminate Z backlash issues that would be a problem for accurate probing. It would eliminate issues with the board changing it's shape once you start milling it (due to stress relief) and eliminates the step of probing itself. All of which may not be problems at all. Assuming running over swarf isn't a problem then it guarantees the depth of cut regardless of what the board is doing. Sort like how a thickness planer works so well on a piece of wood.
As for the price. I wouldn't by one for that price either but it would be a simple enough item to make (for me).
I just thought it was an interesting approach. It might have a whole bunch of problems of it's own that I might like to hear about.
Sage