I've been thinking about this a lot lately. The point of PoW in PPC is as a Clock, and an initial coin minter. Without PoW PPC does not know when 10 minutes has elapsed. Checkpoints are there to ensure that nobody does a double-spend before the network gets big enough to defend itself, as well as provide a clock of last resort (IIRC) I believe that sunny-king did mention removing check-pointing in and upcoming build. (note, I'm not reading the code, using my memory of descriptions)

The Clock aspect of PoW makes it almost impossible to move away from, or at least replace. Using an external timeserver would open up all sorts of network vulnerabilities and attacks, same with checkpoint servers, or any other centralized mechanism.