There are a lot of different reasons for stitching in the ditch. It stabilizes the quilt before you add more decorative stitching. Patches that are outlined by ditch stitching tend to stand out more. Ditch stitching doesn't show, so it enhances the patchwork without overshadowing it. And stitching only in the ditches means we don't have to think up more elaborate designs.
All of those are good reasons to stitch in the ditch, but not every quilt requires it IMHO. If your quilt is well basted, and if you have another quilting strategy in mind, then don't feel that you have to SITD. Sometimes you want the back to have the same quilted texture as the front, and ditch stitching could interfere with that. If you've pressed your seams open then there is no ditch to stitch in. And if you just can't stay in that ditch, you may want to try a different approach, such as quilting beside the ditch or using a different design entirely.