How about this: by keeping everyone in a more central location until you are actually ready to board, I'm guessing you can build a terminal with a smaller footprint. Instead of having a lounge area at every gate that is sized to hold an entire A/C full of people, but then is unused for 23 hours a day so that all those seats have an average of maybe 3% utilisation during the day, you have a centralised lounge capable of holding everyone no matter what gate they are going to. Then you can have a smaller number of really big WC's, plus a centralised shopping/eating complex, and a much higher % utilisation of seats in the seating area. In a larger airport, this is bound to save you acres upon acres of wasted space.

The cynics claim that the aim is to keep people near the shops. Maybe that's just one benefit, albeit a profitable one.