There are just SO many times that the machine bed is not visible when you need to sew a quarter inch seam, especially with a walking foot! Binding, echo quilting, top stitching are a few that come to mind right off the bat. You really need to be able to have the guide above the fabric, not below it, for those times! My walking foot has two slide in guides for grid quilting, but you can't adjust them small enough for a quarter inch seam and there is no marking on the foot itself.
(I'm betting the guides on your Pfaff are metric, bearisgray, like those on my Viking.)