Hi Ken,
My 2 cents:
I use a backplot simply as a visual check for anything that might be really out of wack. It will also give a visual confirmation of where the Rapid motions will occur, so you can check for interference with clamps or whatever. I haven't advanced to the stage where I model my clamps yet, but I'm thinking about that too, because I could make use of Onecnc's rapid gouge detection this way.
Conceivably, the backplot could also show you an area that you completely missed for some reason.
Admittedly, many toolpaths are way too complex to really ascertain much useful info from the backplot because it is such a dense wireframe.
One of the exciting developments coming up is the tool path preview showing a scale model of the tool travelling along the backplot wireframe. This movie simulation will allow you to "preplay" whatever processes you have set up in NC manager. This will give you a chance to catch any obvious blunders that you made in setting up your machining methods before you send the code out to the machine.
This overlaps somewhat with the current simulation function, except that in the current simulation, the emphasis is on the machined surface in "stop frame mode", whereas in the tool path preview, the emphasis will be on displaying the tool motion in an automatically generated movie. I've already seen this function in action (in beta tests) and it simply blows me away. See everything but the chips flying