Bob, I suppose I'd better pick you up on your dt comment first. While I do think it is an amazing resource, I can see no way that it could be considered authoritative.

On to your question. You have mentioned the legal issues but there are practical issues on top of that. From the viewpoint of hosting such a site. Text formats (lyrics, songrwite the dt tunes are written in) and MIDI are very compact. Even if you use MP3, you would be looking at Megabytes per song rather than a couple of Kb per song. That could raise both storage issues and bandwith issues.