If you want to see what happens with no checkpointing with a low difficulty level on a PoW coin turned PoS, check out SHACoin
That coin is forking all over the place. People are syncing on multiple SHACoin chains, not able to transfer coins to each other, exchanges are putting SHACoin on hold, it’s a complete mess.
The point about Sunny’s checkpointing is that it is not centralized. The checkpointing can happen from a decentralized point. Sunny King can be anywhere on the network, connected to any peer grouping, sign a checkpoint, and the network will see it.
So checkpointing does not really mean centralization.
Go look at SHACoin like I mentioned. Read the bitcointalk’s SHACoin thread.
I hope Sunny King keeps checkpointing in longer “just in case”.