Hi Kevin,
I hear you. I think diffraction is ignored because most photographers have never seen a side-by-side comparison of two images where one was taken at an f-stop that provided just enough depth of field to make a three-dimensional subject space appear uniformly sharp, with the other one taken using an aperture small enough to induce visible diffraction. At a given enlargement factor and viewing distance, it's very easy to detect a defocused Near or Far point in the subject space (insufficient depth of field) because subjects that lie closer to the plane of sharp focus are right there, in that same print, for comparison.
Everyone has seen prints with defocused elements surrounded by subject matter that is sufficiently focused. I submit that everyone has also seen prints that were degraded by diffraction - they just don't know it when they see it, because the effect softens the entire image more or less uniformly (disregarding for the purpose of this discussion that Airy disk diameters vary with the color of the light) -and- they never have a diffraction-free version of that image lying alongside for comparison.
Stopping down in the quest for depth of field, most photographers have, on many occasions, ignorantly forced diffraction's Airy disks to diameters that actually exceed the diameter of the largest circles of confusion produced by defocus. It is pointless to shrink your circles of confusion by stopping down if doing so will make your Airy disks even larger.
I submit that most depth-of-field aware photographers don't really know what aperture is necessary to obtain only "just enough" depth of field at the time of exposure. It can't be done without anticipating both the enlargement factor and the viewing distance of the final print prior to making the exposure. (See my last post.) If you don't know in advance precisely how large a print you will be making nor how closely it may be scrutinized, then you had better shoot for the worst case - the largest print your camera's format can bear, for viewing at a distance of 10 inches.
Failing to pre-visualize your final product before exposure can still hurt you even if you shoot for the worst case (big print viewed closely). You'll find yourself always working with very little depth of field (to force small CoC's) and no freedom to use the smallest apertures available on your lenses (to force small Airy disks). If you just continue to shoot as you always have, with no concern for print size and viewing distance at the time of exposure, the majority of your exposures will fall into two categories: Insufficient DoF for the print you end up making and viewing -or- too much DoF in the final print.
What's wrong with too much DoF (assuming your intent was not to use selective focus as a compositional tool) ?
1) As already mentioned, you can induce a softening of the entire image, by stopping down further than necessary.
2) Stopping down further than necessary will also force you to use a shutter speed that's slower than you could have used - increasing your vulnerability to camera and subject motion.
I understand that working in the field with a DoF calculator (like the customizable DoFMaster disks) is just too cumbersome for some shooting styles or assignments. But I can testify it only takes about TEN SECONDS before each exposure to use a spinning disk calculator like the ones you can produce with Don Fleming's freeware:
http://dfleming.ameranet.com/
I shoot fairly static subjects, always working from a tripod, but it just does not take long to ask myself, "How large a print do I want to make for this subject? When I've answered the question, I know which calculator to reach for. Ten seconds later, I'm ready to make the exposure. I've got three calculators for each focal length I carry. (For three different print sizes.) With a permanent marker, I've also written the diffraction-limiting f-stop on each calculator - the f-stop below which I should never stop down for that size print. (See my last post.)
If you can do a little "homework" up front, you'll be equipped to make exposures that produce really sharp prints.
Mike Davis
http://www.accessz.com 
