Is this a strong disincentive to excessive raiding? There would need to be more in terms of diplo penalties to address the raiding exploit.
Suppose 25% off the max defense for each raid. 4 raids = 0 defense, which means that (I would guess) unless you have a ship already at the haven there would be no way to save it from an attack. Even at 50% the haven would not survive for long.
There are lots of havens, but not that many research centers and no new ones are built. As research centers fall the research rate falls for that faction. If PP is trying to win via that factions victory it will have to do more of the research, thus lengthening the game. A key advantage PP has over the factions is the ability to mix/match the fruits of the faction research. The Pandas also gain from faction research.