What hcs said. Me & My Katamari has the best tracks (imo) from the previous two games, making the ultimate collection in my eyes (though no where near to complete).
In any case, the audio tracks from the previous two games are in another format entirely and the tracks are not identical (many of the Me & My Katamari dupes are shorter I think...).
Plus, I love the soundtrack so as many excuses to have multiple copies of the songs in my playlist as possible raises the chances that they will play. =]