It is if you expect it to be honored objectively by offline nodes (propagation isn't objective proof because it can be Sybil attacked). I will let monsterer explain that to you, if you don't understand.
Nodes can identify themselves by cryptographically linking their IP address with an account (called "hallmarking" in Nxt) which mitigates Sybil attacks. The client can be configured to only download blocks from a subset of nodes linked with known accounts and/or a minimum stake threshold. In an attack scenario this would allow offline nodes to filter out malicious nodes and sync with the proper chain. It's basically an optionally enabled reputation layer for trusting nodes linked to known accounts, not a silver bullet because of the flaws with all reputation systems (long term planned exploit) but still helpful as another layer of security.