I used to think so too then I had a c-section and I was willing to try anything to flatten my abs again. The basic plank worked and then I got certified and learned dozens more challenging planks that not only flatten your abs but burn tons of fat too.
If you're an advanced exerciser I highly recommend the
swinging plank or the suspended pendulum plank featured in my
hot abs interval training workout. Those types of planks are hardcore. Not only do they recruit tons of transversus abdominis, the deepest layer of ab muscles that acts like your natural girdle they also burn tons of fat and work your arms and back too!
Basic planks are great for beginners but once you master it you must progress to more challenging versions to keep getting results. There are hundreds of planks you can try. I'll write more about this in the bodyweight article so thanks for posting.