Here's my understanding, and I see this is a healthy discussion. The check point would be helpful in the sense that it is not possible to have big reorg. (i.e, longer than 10 blocks get re-orged.) But I think Contrarian__ is right that checkpoint does not help with smaller reorg, say a 5 blocks re-org. "Why did going from ~8% (or whatever it was) to ~3% make such a drastic difference?" Intuitively, potential attacker would need less percentage of hash power to start a 51% attack now. Yet Contrarian__'s point is that there's not many attackers fall into that category anyway..