Because of how objective based gameplay tends to work out, even in wide open fields, most players should be piling on top of each other around a single point at any given time.
This is my thinking as well. If you have more people playing you either need multiple objectives or some other means of spreading people out. It's certainly doable, I remember some awesome games of Starsiege: Tribes with large numbers of people, but it requires some heavy alterations to the gameplay.
I think 8v8 is a very solid number. If Brink does well adding a new larger play mode, either as DLC or a consideration in a future title, might make a lot of sense. However I'm happy with 8v8 at the moment.
(Another consideration for larger games: bots. If you could have 16v16 with only a few human players the computer would need to manage a tremendous number of AI companions.)