What about those who suggest that crunches are counter-productive, Alex?
I've heard that before, and true, you can overdevelop them if you put too much into directly working them. That's why they have mirrors in gyms, for visual feedback.

And that the abs are worked sufficiently through other movements and exercises?
My opinion is that it depends on what activity / sport you're involved with. I would agree its probably less of an issue for some, but if you're lifting on a consistent and regular basis, you'd better work on keep up that core strength, and not just abs, but all the way around the torso. Most think of front, even the sides, but neglect the lower back - i.e. hyperextenstions and/or stiff legged deads. If you don't, you're going to run into trouble down the road while squats, etal. I find it even helps my form on the bench, preventing me from unthinkingly arching my back (a big no-no).
Aren't abs just a function of overall fitness, lean-ness and where a given person's body likes to store fat?
Yes, and no, I would agree that for some its a matter of genetics (those lucky bastards and bastardettes), but on the other hand some that are very athletically oriented, and have a tight waist, but don't have a discernible six-pack (probably holding water and sodium). Others who'd probably never w/o in their lives and are just naturally lean, having one. Mighty look good, but they're usually not very strong, might not even be in terribly great shape, either.