In order to have instant power-downs + decentralized governance, you need a fee to exit. That is pretty much it. If there is no penalty for getting out early, exchange attacks would be very easy.
So the exit fee is first and foremost a security measure that enables the ability to power down instantly. Now the next question is, what to do with that exit fee? We could burn it, but I think the better idea is to give it back to those that stayed powered up, to reward the ones most loyal to the network. You give short term speculators a way into the Hive ecosystem where. Who knows what happens when you get whales with big egos splashing around Hive upvotes? It could be an entertaining event. And when the short term speculators have had their fun and move on, the long term investors are rewarded.