The way I understand it is as previously stated, quilting in the ditch is actually quilting on the seam weakens the quilt because you're simply going over the seam you used to put to quilt together, whereas quilting beside the ditch is yet another seam which strengthens the quilt which is what quilting is all about.
I'll get a picture of the inspiration quilt if I can figure out how to post it so you can see what I mean.