Not on Win7 probably because of Modern UI. WP7 - it's getting phased out anyway. The Zune service is supposed to be available for those 2 devices, however.
Didn't they say they were going to release it for iOS and Android next year?
Though, I do think there needs to be a beautiful desktop app for managing music. The latest iTunes is great (from an aesthetic standpoint). The desktop is for sitting down and cracking away at the details -> music enthusiasts would want to be able to sift through their collection, edit metadata, etc.