Received this question the other day:
Do you think "ab only routines" are worth doing?
This is actually a topic that has been debated for some time now. On one side of the fence, you have folks who think that you get plenty of core work through squats, deadlifts, etc. and they're right. Science has shown that core engagement, especially the posterior chain, is much higher and will lead to a stronger core if you do things like squats and deadlifts (Romanian deadlifts as well). Unfortunately, the "abs" aren't as engaged during these exercises, however the actual abdominals are not nearly as important as some believe.
The other side of the fence believes that you should work your core similar to how you work other body parts, to not only improve overall athleticism and performance, but to help prevent injuries. I fall somewhere in the middle, yet a bit more on the side of the fence believing that core-specific work should be done to aid in overall health. Notice how I call it core work. That is why I have chosen some of the best exercises for core engagement and incorporated them into my Core-mageddon routine.
In my personal opinion, individuals who do too much ab-centric work (crunches, etc.) can actually lead to back pain. This can be caused by having tight/strong abdominals - you are great at crunching your midsection, but that can therefore lead to a posterior pelvic tilt by pulling your pelvis out of alignment.
Although I may sound like a broken record, let me reiterate what I have been saying time and time again; fat loss is not area-focused. Crunches will not give you a visible six pack. A well-rounded training program, coupled with a healthy diet will lower your body fat to a point where your abdominals will become visible.
Total body strength training, using exercises such as barbell squats and deadlifts will engage and improve the strength of your core tremendously. You may find that that is enough for you. If, however, you wish to improve the strength of your midsection then I recommend a core routine that improves the functional strength of your core rather than just aiming for aesthetic gains.