Darkness Falls was definitely a PvP balancing tool. You fought hard in PvP to gain realm-wide bonuses (for those not in PvP as well), and in addition you fought to open the coveted dungeon. The winning realm would high-tail it to Darkness Falls, thus leaving the PvP frontiers open to those in 2nd and 3rd place to square things up. Sometimes Darkness Falls could remain in one realm’s control for a few days. Sometimes the other two realms swooped in and used the opporunity to take many keeps and towers, thus turning the tables in hours or minutes.
I remember getting the call over Alliance: WE HAVE DARKNESS FALLS.
People ran from all over, dropping everything to get inside. There was no cap on the population. If 200 or 1500 people got in, they got in. It was large enough to handle the load. It was exciting to know that enemy inside Darkness Falls would be there and no reinforcements would be arriving for them.
If ArenaNet offered a reward for WvW points, other than wang swinging and forum flaming, it would also help level the match-ups with disparaging population imbalance.