Blogs that have plenty of (good) images can benefit from such function. For example MediaRSS feed can be used in wallpaper changer app.

It can also be used in syndication for cross-promoting content. I am including my latest stumble in my footer with graphic thumbnail and it would be so much easier if feed was MediaRSS.

Other than such specific uses my guess is rest of bloggers couldn't care less. :)

PS oh and I wish before this fancy stuff someone fixed rss.php in WordPress to support enclosures