In Guild Wars 2 for example, the travelling from Keep to Keep is so fast that there is little to no point in splitting up your forces. Rather than splitting up, its much more beneficial to simply run as a big zerg together since the Keeps are only about 1 minute run from each other. The fact that everything is so close and that players can freely teleport to nearby friendly Keeps also contributes to this zerging mentality, as there really is no "need" to separate your forces.
In GW2, if players need to go Defend or Attack a Keep that is really far on the other side of the map, they simply order the entire zerg to teleport to the nearest Friendly Keep and run the remaining distance within a minute or so.
While huge massive zergs can be fun, it removes much of the strategy aspect and after a few weeks of doing WvW in GW2 where all you do is run around with a massive zerg, it can start to get quite boring and repetitive.
I remember watching an interview with Zenimax in which it was stated that players can teleport to friendly Keeps in Cyrodil. I just hope that despite this fast travelling method, that there will be something in place to discourage this "One big zerg" mentality and play style. From experience (playing GW2) I can tell you that this takes most of the fun out of the game since no matter how far one of our Keeps / Towers under siege is.. we know that the entire zerg can just teleport nearby and make it there before the first gate goes down. No one ever stays behind to defend Keeps or Towers since that's also rendered pointless due to how fast every single player on the map can get from point A to point B.