1 - a given airplane doesn't fly the same route exclusively. So a 319 may fly transcon today and Shuttle flights tomorrow. Same for the 737, though obviously you won't see them on transcons - the one going CLT-MCO-CLT in the morning may be doing CLT-ILM-CLT that afternoon. So loads vary for each airplane and each airplane type from flight to flight and day to day.

2 - US actually has among the lowest load factors of the network carriers.

Neither of those are to say that any flight isn't oversold some of the time, because it definitely happens. When it does, having more seats means selling more seats.

My personal feeling is that there are 3 reasons for reconfiguring the airplanes, in order of importance:

1 - reduced CASM because more seats means the airplanes will produce more ASM's with basically the same cost.

2 - incremental revenue when more seats can be sold

3 - standardization for the coming integration of operations, after which most any 319 (320, 737) can show up anywhere. I put this last because it doesn't seem important enough for management to reconfigure all the airplane types to the same configuration (i.e. - the 737's and 757's).

Jim