The way I understand it, you need to give your driver a command once in a while to keep him focused.
Think of him as a real person, and you're the guy with the mic on the stands. You can assume that your driver will do a better job if you talk to him now and then, keep him updated and tell him what to do. Imagine that you are the driver. You would feel better, because you know that there's an extra pair of eyes and ears virtually racing with you, wouldn't you?
If you were to tell him to keep the pace and then turn dead silent for 15 minutes however, it might make your driver uncomfortable and lose focus, which is pretty much what happens in B-Spec.