I looked at these for a while, but really you're just following a template on the opposite side of the workpiece with a pin the size of the bit instead of a template guide on the same side of the workpiece with a guide 'slightly' larger than the bit. Plus, you are adding whatever movement exists (no matter how small) between the base, stand, and arm as a cumulative error when following your template. You still have to make an accurate template, even if it is duplicating an original piece. And if your original has roundover edges, your chances of accuracy decrease even more.
I decided it was just as easy and 'possibly' more accurate just to make an MDF template pattern in the first place and use template guides. And I know that the pattern/bit relationship will not change as I am moving around the edges.
Just my .02, your milage may vary.