Annotations and properties share an etag. Copying the etag from annotations into properties or properties into annotations avoids a (likely) unnecessary round-trip to refresh the annotations, which (likely) haven't changed.

I don't think propagating the etag from properties to annotations hurts if you're interleaving update operations between two users. One user will get an 'etag out of data' error. That might happen in between operations -updated properties, failed to update annotations. That issue is minor and should be solved by EntityBundles.