Let me turn it around. With a global bitxoin system, what is a good reason that nodes can not freeze all blocks older than say 10 hours of accumulated difficulty. Where is the downside? Is it really considered a desired feature that a part of the network can be hidden for many hours and come back again with a long chain? Or is it just that checkpoints
haven't been implemented because the system is still so new?
Under such a system, a bad chain (such as the one we fixed yesterday) would get locked in if it lasted more then 10 hours. I'm sorry, but automatic checkpoints are a bad idea.