A more sophisticated free-rider attack involves taking advantage of the grace period to obtain backup service for free. The attacker joins the system, forming partnerships and exchanging data as normal. He then pretends to crash, throwing away all the data he has been given. For the next 2 weeks, he has free backup service because of the grace period. Just before the end of the grace period, when his partners will stop giving him backup service, he switches to a new set of unsuspecting partners and starts again. So long as he can find new partners (peer-to-peer systems can have millions of members), he can continue to receive backup service without cost.
Another free-rider attack involving the grace period has the attacker refusing to wait out the grace period before abandoning partners that are excessively down. This hurts his partners because they are left with a less redundant backup, or even no backup at all if enough of their partners free ride this way; however, the attacker benefits because his backup has better redundancy for the next two weeks because of the additional new partner.
The only way to deter these attacks is to make them unprofitable: we need to arrange things so that the attacker pays more for the privilege of using the grace period than it is worth to him and so that the attacker saves money by waiting out the grace period without abandoning his partners. (Free riders are motivated by the chance to save money, not the opportunity to hurt others.)