one advantage is that it reduces the risk of tail tipping if there is a blockage de-boarding in the middle of the cabin and a lot of pax ( weight ) is stuck at the back with all the pax gone from the front of the cabin.
You got a source for this? QF have been de-boarding a fleet of up to 75 B738's via front door for the past 15 years without any tail tipping events! Although sometimes the nose oleo lifts quite a bit with 80 pax still standing rear of the overwings. I think there would be a serious certification issue if there was a risk of tail tipping...
EDIT: I have heard of one in Malaga but that was apparently caused by a rogue aerobridge that lifted the nose of the aircraft by lifting the L1 door. Auto level fail or something. I am also aware that there is a cargo unloading policy in place in QF to ensure that front cargo is not unloaded prior to aft cargo. I don't know if that is a QF policy or a manufacturer policy. But I have not heard of any issues related to pax boarding/de-boarding.