In theory, you're right. The problem is that theory does not take into consideration things like voltage drop, poor connections, damaged devices and wiring, etc. The service coming into our homes doesn't have separate grounded and grounding conductors. In fact, the neutral itself is what defines electrical ground. Same for older 3-wire major appliance circuits, where the neutral was permitted to also serve to ground the appliance (and not the other way around).
First, electrical devices with exposed conductive parts need to be grounded so an accidental internal contact will open short-circuit protection equipment. Second, a poor connection in a grounded circuit conductor could allow wiring that is expected to be at zero volts to become energized. Third, allowing a grounded circuit conductor to serve as an equipment grounding conductor means exposed conductive parts could become energized, creating a shock hazard.
Years ago, a customers son was severely injured when he touched the range and sink simultaneously. I found that the neutral of the 3-wire cord had pulled out of the crimped-on ring terminal, energizing the range cabinet through the clock and lights, which stopped working a few days prior. There was 120 volts between the range and ground, and 240 volts between the range and one of the lines. The now-required 4-wire circuit would have prevented the shock.
The only thing the jumper does is allow the receptacle to test as if it was properly grounded. Again, yes, in theory, the jumper seems like a better idea than no ground, but it can introduce possible hazards that having no grounding conductor won't. Remember, electrical systems have evolved over the years. We had no way of starting out with the systems we have today without having gone through the experiences as needs and equipment have grown.
