My experienced has been that pulling with a rounded lumbar spine does not typically lead to injury. What does lead to injury is setting your back in extension, and then for whatever reason, letting your back flex in the process of pulling the bar. That movement from a shortened muscle being ripped into elongation seems to cause injury that held flexion does not. Sometimes this happens because the lifter is jerking the bar off the floor. Sometimes it happens because the lifter has poor positioning (bar forward of the midfoot). Sometimes it happens because they lose focus on maintaining an extended lumbar.
However, when your back is extended, it is incorporating your lumbar extensors (which really are postural muscles used in isometric contraction) which is stressing them, which allows them to adapt and get stronger, and then you don't get hurt playing with your kids or moving your lawnmower when it gets stuck in mud or picking up a couch.