A while back, I toyed around with posting an anti-crunch blog but I decided to delete it. As much as I'm a BW guy, I almost never do a crunch for exercise. I've never liked them. The closest exercise that I do to a crunch is a V-up. I decided against running the blog since I didn't feel like I knew enough about why I didn't learn the crunch to criticize it intelligently. I may not be an expert but I try to be knowledgeable.
Well, after a year, I believe that I do. In fact, my eyes have come across several sources that don't think highly of the crunch for abdominal work. I read a book that told a story about Vince Gironda throwing a client out of a gym for doing crunches.
Initially, people thought that the abs were supposed to lean the torso forward. It make sense considering the insertion and attachment points of the abs and muscles pull their ends together. The truth is these muscles are supposed to contract intensely when the spine gets loaded. So, good ab exercises shouldn't cave the stomach in. The stomach should be flat when contracted. Caved-in abs are weak.
The best ab exercises are the ones that force your abs to contract while maintaining that flat shape. Exercises such as dragon tails, ab rollouts, inchworms, most push-ups, planks, L-sits, etc are the, in my book, the exercises that you want to be doing instead of crunches. These are the functional exercises that will serve your body best. You'll also go a long way to protecting your lower back if you select these over crunches.