It totally depends on the individual. I believe for many people situps are not good for the following reasons:
-put strain on the lower back
-strengthen the hip flexors
-get people used to entering hip flexion when they should be entering spinal flexion
-get people used to using any means of completing an exercise, momentum, any muscle that could possibly help..
They are good for the following reasons:
-Strengthen the hip flexors
-Standard of work. a situp is obvious to see. a crunch is much more subjective
Most people can benefit from doing situps occasionally as part of a program. They should not be the base on which you build your core training but they have their use. If you have strong enough abdominal muscles to keep a C shape to your spine while entering through the midpoint of the hip flexion phase of a situp, they can be a very demanding exercise.
But as another poster stated they can increase the anterior compressive force on the spine, maybe even when done correctly...