My Top 5....

....reasons why crunches are a waste of your time:

1. In order to get any sort of training effect you have to do hundreds of them since the range of motion is so short.
2. Your spine doesn't enjoy repeated spinal flexion. Constantly sending compressive loads through your spine is setting you up for an aching back.
3. In the era of "functional" training, they're about as functional as using the Smith machine...not functional at all. Think about it. How many times a day do you call on your abs to flex? Once, when you get out of bed in the morning. A second time, if you're getting up from a nap.
4. Some of the most prominent researchers and therapists in the rehab field have suggested the role of the rectus abdominus to is resist trunk extension, not engage trunk flexion.
5. They're doing nothing for your waist line. If that's what you're searching for, you're better off doing 10-12 minutes of high intensity interval training.