In the prerelease thread it says there are centrally broadcasted checkpoints to defend against 51% attacks. That thread was started in 2012 and the codebase may have changed since then, but has a new way to defend against 51% attacks been implemented? Is that why users can now choose to disable checkpoint enforcement?
https://bitcointalk.org/index.php?topic=99735.0Centrally broadcasted checkpoint for transaction settlement and defend against 51% attacks
I believe the original idea was to implement checkpoints until PoS matured on the network and could act as a defense mechanism if PoW was ever attacked. It has now had years of maturation.
PoW doesn't play a role on Peercoin network security. PoW is only for fair distribution.
Peercoin security solely relies on PoS. Peercoin PoS security has been improved in Peercoin v0.5+.