I think there is a difference between what happens at foot strike and what happens after foot strike. I think guys like Ryan stride and plant with a fairly straight front leg but then allow the front knee to bend before firming it up. Tell me if you agree with this:
Whether you land on a straight or bent front leg is a comfort thing that varies from pitcher to pitcher. Regardless of which you do, after you plant your front foot, the total body must continue to track forward while shoulder rotation is delayed. We want the shoulders to rotate around an upright spine so the torso needs to be kept upright while the body tracks forward. This is more easily accomodated by either landing on a bent front knee or allowing the front knee to bend right after landing on a straight front knee. In either case, though, the front knee must then firm up to slow down and eventually stop the hips from moving forward to create a more explosive rotation of the shoulders. Pitchers who land on a straight front knee and never allow the front knee to bend will be forced to bend forward at the waist and won't be able to rotate the shoulders around an upright spine. They will tend to get their torso too far ahead of their lower body and won't be able to optimally use their body to throw with thereby putting more stress on the arm.