Why are downloaded music files stored in a hidden directory?

The Music Store downloads to a specific folder which is individually synchronised with Ubuntu One (a new feature in 10.04 Lucid), rather than to a subfolder of Ubuntu One. For technical reasons, it's not possible to have one synced folder inside another synced folder, and lots of people sync their existing Music folder, which would therefore break if we made the Music Store folder a subfolder of Music. So, the actual folder is hidden in .ubuntuone, but a symlink to that folder is created in the standard freedesktop data storage area for ubuntuone (likely to be .local/share/ubuntuone on your machine) which has a translatable name so that it can be made available in the user's choice of language.