Dicken's Bob Cratchitt probably worked at a slanted, standing height desk. However, Allan Hedge certainly deserves great credit for reminding us of the value of tilting keyboard tray surfaces. As we encourage, or even demand, that keyboard workers change position, the equipment we use needs to support those different working postures. Some infomation that the Steelcase furniture company recently circulated suggests that office workers change posture many times per hour. There is no question but that people have to be trained to use equipment appropriately.
Many keyboards still have the flip-down legs that change the rake, or tilt of the keyboard.
If we restrict the tilt adjustability, we drastically restrict the flexibility to deal with changing situations.