Any quilting, including SITD, is not meant to hold your piecing together - your piecing stitches should do that. The purpose of quilting is to hold the 3 layers of the quilt sandwich together, in such a way that nothing shifts. (I have, however, used quilting stitches to hold together a vintage quilt that was coming apart at the seams. That's not how it's supposed to work, but in that case it wasn't worth the time to try to fix the real problems - seams that were too narrow, fraying fabric, etc.) Sometimes SITD is done before more intricate stitching, as a means of stabilizing the quilt. It only works when there's a ditch, meaning the seams are pressed to one side, and is especially hard to do if the seams aren't consistently pressed. It's just another technique for the machine quilter to use, and not the easiest, especially on the longarm, but it can be very effective. I found scissor queen's comment about the origins of SITD very interesting, something I hadn't known before, and it totally makes sense.