Right now the post thumbnail is an attachment, a image file in the uploads folder. But what if there 1000s of posts and you don't want to have 1000s of thumbnail/image files saved on your server? Or you want the thumbnail to be some nifty (possibly offsite) URL, maybe including a query in the URL? So please! redesign the post thumbnail to allow it to be an arbitrary URL (either onsite or offsite), and enable the 'Use as thumbnail' in the 'From URL' tab of the 'Insert image' SubPanel.
By extension, this idea also applies to any image or thumbnail on wordpress, for example the attachment thumbnail, design it so it can also be an URL.