It would be useful to be able to know whether an entity's thumbnail was set via the share_thumbnail() API method, or not. Scripts that share thumbnails between entities may want to detect if a thumbnail was updated manually already, and if so, choose not to override that via an automated sharing method. It may also be useful to be able to tell if a thumbnail has been shared with other entities, as you may want to update the other records that share the thumb in case it is updated at some point.
Click 'Me Too!' to vote up this feature, and add any additional thoughts or use cases in the comments.
