I ended up having an MP3 Locker because I had bought some music at MP3 Tunes. At first I didn't understand the value of the service and just found it annoying (the script in the plugin cannot deal with big pages full of MP3 links for example).
But recently I've been reconsidering. I have been visiting some of the various John Peel style blogs that are either dedicated to the program and the music of the time or honouring the eclectic style of exploring new music. Most of these blogs have links to recordings of old radio shows, obscure 7" or band sessions. I don't want to download such eclectic material onto my PC, it would be a pain to handle and if its a duffer I don't want to lose it amongst all the other files I have.
MP3 Locker means I can download the file without having to store it on my machine. It also allows me to store a link that may be ephemeral. The locker also has a nice web based MP3 player and allows you to have a quick a blast of something while wandering the web.
It's probably not 100% there yet but it has been becoming increasing useful, to the point where I actually noticed that the SL icon was missing on my MacBook. For music grazers I would definitely recommend, it's the junk store of online music.