I was asked this question last night. My answer was two-fold.
First, flexing/bending the spine with a strong muscular contraction is a recipe for disc problems and low back pain. Most people have poor posture, including a foward-flexed spine, already. Adding a forceful bending, twisting motion to the spine is asking for problems. Those of you with disc problems have a small margain for error and know what I am talking about.
Secondly, in order to move well you need to restore functional stabilization to the spine. That means getting all the muscles to do their job (i.e. Keep the spine from buckling) though a variety of motions. Another way of saying this is that the "core" actively transfers force between the upper and lower body, and vice versa. We use a variety of "challenges" in multiple planes (not just lying on your back) for the body to maintain spinal stability throughout. I like to think of spinal stability as a skill that can transfer over to many other movements.
Here are two examples of these "challenges."
1. Pikes in the rings are a type of anti-spinal extension exercise. All the movement is in the hips and shoulders while the torso stabilizes.
2. The unilateral farmer's walk over hurdles. Again, the challenge is to maintain a neutral spine as your hips move and you carry an off-set load; a type of anti-lateral flexion.
These are just two examples of many. The body usually initiates movement with the limbs and transfers force through the torso. Sometimes, it does just the opposite. However, when you develop solid torso stability you really gain awareness and spinal control. And when you do initiate the rare movements from the torsro, you are stronger and more controlled.