Any subnets cutoff from the main network would have to operate under an assumption that a) their blocks may be invalidated and b) they may have to take manual action to rejoin the main network. The subnet could continue to mine to ensure the integrity of their own transactions while separated from the main net, but they would have to expect that their block rewards may eventually be rendered invalid.
If the 6 block auto-checkpoint were contingent on being connected for 24 hours with a normal incoming stream of blocks at a normal-looking rate, someone separated from the main net would be spared the need to do anything manually to rejoin. This would be because while they were separated, there would be a big gap between the difficulty and the rate they were seeing blocks, and this would disable their checkpointing.
Above all, it would make us look much better to the media: "Bitcoin Attacked, Contingency Plan Activated, Disruption Minimal, Just Click Button" versus "Bitcoin Hacked, Nobody Saw It Coming, Developers Racing Around Clock, (Untested) Patch Expected Soon, Maybe Bitcoin Was A Bad Idea From the Beginning After All"