That is very cool. The Rock Band series of games, which I've never actually played, have made it possible to hear tons of tracks from tons of songs... the games apparently come with multitrack Ogg files. People figured out how to extract the tracks from the game DVDs and often they show up on youtube (search for "isolated bass" and a lot of what turns up is from Rock Band). This guy probably got his material from the same source. But putting it on an easy-to-use interface like this is a clever and really useful idea.
Personally I think the availability of the Rock Band tracks, and now this web thingy, is a fantastic development. You can learn a lot about how arrangements are put together and how the individual details relate to each other. And how parts you think you've known your whole life are actually quite different.
Now if only somebody would get hold of tracks for "Funkentelechy"...
