^ Pretty much as already said above. 5 groups of 8 players will easily outscore a zerg of 40 people simply by the virtue of being more maneuverable and being able to cap more places simultaneously. There will be times to form very large groups to assault the larger keeps but after the mob should split up in order to cover more ground.
There is a balance between solo and zerging/mobbing. Hitting this balance point allows most efficient use of available players, this will decide many WvW matches imo.