MigrateDestinationEntityAPI::getKeySchema() adds revision key to destination keys. Why? [#2765495]

Hi,
I found something in the code that surprised me..

In class MigrateDestinationEntityAPI:

  static public function getKeySchema($entity_type = NULL) {
    [..]
    $revision_key = isset($info['entity keys']['revision']) ? $info['entity keys']['revision'] : NULL;
    $revision_schema = empty($revision_key) ? NULL : $schema['fields'][$revision_key];
    [..]
    $return = array($key => $key_schema);
    if (!empty($revision_key)) {
      $return[$revision_key] = $revision_schema;
    }

I see no other implementation of MigrateDestination where the revision key is added to the destination keys.

Why this is happening?
And what does this mean for the two cases where
1. I want a new revision to be created every time this is imported
2. I want the existing revision to be updated.