After going through the codex pages for all thumbnail related pages I didn`t find a way to limit the thumbnail creation to a specific post type.
let`s say you have a custom post type for "books" and thumbnails are 260x260. WordPress creates the 260x260 jpg for every uploaded images where it should only make sense to create that specified size for images uploaded within the BOOKS custom post type.
add_theme_support( 'post-thumbnails', array( 'books' ) ); add_image_size( 'book-thumbnails', 260, 260, false );
having another parameter for add_image_size which works similar to dependancies (wp_register_script) would help to limit it to the post type.
add_image_size( $name, $width, $height, $crop, $post_type );
I assume the default thumb\medium\large sizes in the /media.php setting are responsible for many never-used images. I would leave these settings completely up to the theme developer who should decide the size for thumbnail\medium\large. And only create these sizes IF they are specified in the theme or entered manually in /media.php
imagine the ressources you would be able to save with an extended thumbnail configuration. (backup space and bandwidth; cpu load, diskspace, fragmentation etc.)
I know a few wordpress users who messed up their upload folder with seveal thousand image files while only using maby 100 or 200 images on the site itself. They didn't even know about it and just wondered why their backup zips are >500MB.