How to define destination for specific file bundles? (and why use separate file migrations?)

I'm trying to migrate a D6 node containing a Flickr photo field to a D7 Media flickr field (using the migrate d2d module).
The problem is that I'm not sure how to define that a field is a Flickr type of Media (instead of Youtube for example).

Normally when I migrate files to Media of file fields I just give the filepaths as value and make sure all the subfields are properly files, see the following snippet.

    $this->addFieldMapping('field_media', 'ps_images_filepath');
    $this->addFieldMapping('field_media:file_class')->defaultValue('MigrateFileUri');
    $this->addFieldMapping('field_media:file_replace')->defaultValue('FILE_EXISTS_REUSE');
    $this->addFieldMapping('field_media:preserve_files')->defaultValue(FALSE);
    $this->addFieldMapping('field_media:source_dir')->defaultValue('files2migrate');
    $this->addFieldMapping('field_media:display', 'field_img:list');    
    $this->addFieldMapping('field_media:description', 'ps_images_description');

In this case I will fill the ps_images_filepath and ps_images_description in the prepareRow().

But this time I have photo Id's that need to be translated to proper url's so I think I need to define it should be handled by a MigrateExtrasFileFlickr kind of class. Any tips?

Most examples use a seperate migration for these kind of files, like the Migrate Extras example for youtube files:

 $this->destination = new MigrateDestinationMedia('video',
      'MigrateExtrasFileYoutube');

But I've never understand why you would do this in a separate migration class and I'd like to integrate this into the node migration. In the first example I don't need a separate migration to migrate the images. Could anyone elaborate on this?