>Maybe it didn't spread because it's not a webapp ?
I think this is it. I remember checking out Tomahawk a while ago, about the time Songbird announced it was discontinuing development. I ended up just going back to iTunes because it did all I needed (even if it is bloated). If I can install Tomahawk on a computer, I can probably install my music library onto it as well, so I don't need the resolving features it has.
I actually just tried installing Tomahawk because of your comment, and it does work very well, and looks pretty while doing it. It was able to find several obscure artists with perfect accuracy. (Though it did grab most of them from Soundcloud at 128kb/s, when a few of them are on Spotify with higher bitrates). But unless I can access it anywhere, with a moment's notice, I don't really have a purpose for it being installed on a machine.
I rarely even listen to music on my computer anymore. I use my phone for Spotify on the go and even at work. It's nice to not have another program running while I'm working. But a lot of artists I like aren't on Spotify, and my phone doesn't have enough storage to fit my entire library.
That's why this interests me so much. I would love to set up a server where I can open the URL on my phone, laptop, friend's laptop, etc, be able to play most of my music through web services, and the artists that aren't on those services would play from my server directly, or my DropBox, or whatever, and hopefully it can all be a seamless experience.