There are two scenarios where an incorrect shardIdentity document can matter.
1) When adding a new shard, if there is already a shardIdentity document on the shard, and it doesn't match the one that is attempted to be added during the addShard process.

2) When the shard is already a part of a sharded cluster, but somehow the shardIdentity document is incorrect and doesn't match what is sent by mongos in setShardVersion.

For both cases we should manually double-check that things fail the way we expect them to, and with reasonable error messages that would be clear to a user and to our support team if a user somehow got themselves into one of those states.