Yeah, the really good soundtracks in Dissidia alone would be reason enough to buy it.
And as far as I remember, you could also use or unlock a kind of jukebox in the game, as well as additional music tracks.
But basically, the soundtrack is just the icing on the cake of the game, which is partly a modern beat 'em up with interactive, destructible arenas and partly a very complex, extensive RPG with an atmospheric story - in other words, a really successful combination of game genres.
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Here you can get a tiny glimpse of how music, gameplay, and story come together in the second Dissidia game, Dissidia 012 Final Fantasy, for the PSP:
https://www.youtube.com/watch?v=BKtTPeUk0yA
And that's just the beginning of the game, which can offer over 500 hours of gameplay until you've unlocked all story parts, game modes, characters, music tracks, equipment, summons, items, and other secrets.
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Given the huge potential of this game, which is far superior even to the legendary Super Smash Bros. series, I'm already expecting a really big next-gen title for the Dissidia series, even if it's just a remake of the first two PSP games for now.