The movement of the crunch depresses the rib cage and rounds the shoulders forward. A lot of people already have overly kyphotic shoulder postures and shortened rectus abdominis muscles (six pack muscle) and crunches are the last abdominal exercise they need to be doing. The lower abdominals and obliques are usually the weakest ab muscles that I see when I start working with someone.
That being said, there are MANY ab exercises that are not only better and more effective in general, but also much better for many individual's needs and for carryover to specific activities.
Planks, side bridges, rollouts, russian twists, standing abdominal work, cable chops, reverse crunches (done correctly), and leg/knee raises and lowerings are just some of the multiple exercises that are more effective then crunches.
Now, I am not saying that there is never a time when some form of a crunch may be appropriate for a certain individual. In fact, Stuart McGill, THE spine and back expert, has a version he calls the curl up that can be effective for certain individuals at the right time. Even when he does use his version, all other torso muscles are trained and balanced out.
If you are like most uneducated people that I see, chances are that you need to drop the crunches, quit wasting your time, and do something that will actually help you. Notice I said nothing about situps, I'll cover those in another post.