We estimate the cost of various types of attacks on proof of stake currencies and compare it with the cost of similar attacks on a proof of work currency. (bitfury.com)
It thus keeps a record of it in the immune system and can quickly deal with similar attacks in future (4). (momjunction.com)
But the upside in that could be that real experience of such an attack would provide the stats to guard against similar attacks on other networks in future. (coindesk.com)