Airlines offering premium service require the pax to use only the forward door so that caterers can access the rear galley to resupply the aircraft in a reasonable timeframe.
I beg to differ, I have catered QF aircraft (737's) with both front & rear doors active for disembarkation! Normally we would use one truck to do both front and rear galleys, but if a flight was running late we would use two, in most cases the aircraft would be fully catered before the last passenger had disembarked.
The number of doors used has more to do with the facilities available rather than the level of service!