For esthetical reasons, there is no point in doing fast abdominals. It might increase the reflex arc, and as such give you a higher tonus and firmer abs though; I've noticed a slight difference.
The main reason for doing fast abdominals is to gain functionality and speed. If you are playing any sports then it will better your performance. I am a professional sprinter and since I started doing fast abdominals my performance has gone up.
What happens is that they learn to contract faster. So when you run for example, then in every stride your iliopsoas forces your spine into lordosis. That is an unfavorable position running because it inhibits the glutes and does not transmit the forces from your upper body and as such makes you less stable. If your abdominals can contract quickly enough, as fast as the iliopsoas pulls the spine forward, then you will lessen the lordosis. The same goes for sidewards movements such as in most team sports and tennis.
Despite being a stabilizatory muscle which demands long isometric contractions to provide stability, there is some function in doing fast abdominals so that your performance increases. I personally do isometric exercises on the days when i aim for hypertrophy in the gym, and explosive ones when I sprint and do max lifts.