I can see where a defender could try and summon purges back to back to block/defer a raid. However, right now there is a 10%+ chance that the purge camp will spawn then despawn because a cage or tent is blocking the camp exit.
A raiding clan could also hang around outside of the invader’s camp, rush in at the end and hope the vendor spawns. That way they could buy loads of bombs and trebuchet ammunition right by their enemies’ camp.
In the livestream, Andy talked about the difficulty scaling of the purge and how if any bystanders participated, the difficulty would increase. I can see raiders picking off a few invaders to increase the difficulty of the purge for the defenders.
In any case, 4 days till 4 days till Chapter 2 is released. And 4.1 days until the complaints of how the new purge is unfair begin.