Leila, the “straight-legged” dead lift is a haphazardly coined name from a loosely-phrased era in weight-training, a rather unfortunate label or misnomer for a purposeful and productive exercise. It’s implication does not translate to what I am certain all of us teach and practice when demonstrating or executing proper technique. For the reasons you so articulately espouse in the answer to your very own question, a 30-degree knee bend should be maintained throughout the movement. This critical element in performing a proper repetition cannot be emphasized enough in order to prevent those new to this exercise from risking spinal injury. Failure in this regard would preclude anyone from maintaining nuetral intra-exercise spinal curvature and could jeopardize structural integrity. At the very least it inhibits the efficiency of the exercise. I like to refer to it as the “stationary-legged” deadlift (to avoid conveying a locked-knee position) while illustrating a proper knee-bend “hold” and pointing out the rotation exclusive to the hip joint.