What's _really_ valuable, though, isn't real-time traffic information, but future traffic information. What matters isn't the traffic right now, but the traffic as it will be when you arrive at a given portion of the journey. I suspect that at present cabbies have the edge there, but that edge will also be swept away by predictive algorithms incorporating knowledge of typical traffic patterns plus lots of inputs about events that may affect traffic.
The latter point is particularly interesting if a navigation system acquires enough scale that it is directing a substantial fraction of the traffic. In that case, it knows not only what external events may affect traffic, but can further make decisions to control the flow, distributing and smoothing.