Well, we aren't talking about a pathology here, we are talking about natural ROM and function of the spine. We talk a lot about "not letting your back round" during a deadlift, and I agree, to a certain extent. There has to be spinal extention and flexion in a deadlift, what we don't want is for this movement to take place outside of willed muscular contraction.
If you lift and your back rounds out because you do not have the stability to hold a weight, that is bad. If you apply a large amount of force (jerking the bar off the ground, or possibly during a hitch) while the spinal erectors are not engaged (slack muscle trying to suddenly "catch" and stabilize a weight), that is bad. But a stabilized rounded back isn't bad, in and of itself. Anyone that has ever stone lifted can attest to the need to gain ROM by allowing curvature in the lumbar spine, unless you have some serious ape arms
You simply can't lift a large ball from between your legs by squatting over it with a "flat back."