Steve - the park frequency is normally a voice frequency. The signal receiver is already monitoring the control channel so placing the voice receiver on the control channel doesn't offer anything.
The one advantage of this setup over a single scanner is true a preemptive scan. The 396T - like all scanners - must leave the control channel to follow a call. If a more important call comes through, it won't know. By using a separate dedicated receiver for control channel activity, the program can preempt a low priority call with a higher one. If your ears and wallet can stand it, you can have multiple voice and signal receivers.
Of course, this isn't nearly as convenient or portable as a properly programmed handheld scanner.