On my 97 Maxima I remember reading in the FSM that yes the vehicle has both MAP and MAF sensors, but the MAP sensor is only used for diagnostic purposes and does not directly affect engine performance. In other words, the primary input for air/fuel mixture is the MAF sensor.
I think in the a 2003+ Murano, this is most likely the case too. GripperDon is correct in that the MAP sensor was once used for this purpose, but today, the MAF sensor is the only input which the ECU actually uses between the two to optimize engine performance.
Why is the MAP still there? I believe it is used today as a part of the emissions control system to monitor for vacuum leaks within the intake system.