Link to the schematic on arduino's website.From my understanding there is a comparator circuit in the arduino Uno schematic which turn off USB supply when there is a Vin present of greater than 7V by switching the PFET off. What I don't understand is why there is the need for the comparator circuit? Is it just to ensure that the Vin voltage is never below 7V? To ease with cost could you not simply plug in the vin into the gate of the pfet. That way if it goes low the pfet would turn on and vice versa? Also the LDO's connected to the Vin won't work anyway when the voltage is below 7V.
Is this to prevent damaging the design if you put loads of volts across the vin and burn the fet?