Great thread, Harp!
I've taken enough screenshots now that I can attest to the general premise. Without very careful planning/execution (and lots of photographic knowledge), I've found it extremely difficult to accurately capture what I'm seeing on the screen. There are methods to get accurate (at least much more accurate) screenshots, but quite frankly I know of only one person that follows that rigorously follows that methodology (and that's mech).
I do think screenshots are useful for showing the relative difference in "range" (brightest white to blackest black...analogous to perceived contrast ratio) that screen A can display vs screen B. Some of Bill's photos in the PFG thread are a good example of that. While they may not be "accurate" in the absolute sense (meaning vs what he acutally sees), I think that it's still possible to gain insight into the screen performance. Even then, scene selection/variety, camera quality/settings, etc etc are still extremely important.