Yeah, if you already have a DS there isn't much point in simply avoiding it. Despite the questioning of its relevancy, it is an enjoyable game more or less. If you don't have a DS though, you might want to just wait and buy a 3DS, so you can play KH3D, Re:Coded AND Days.
It answers a lot of questions and has a pretty good story. And you get to spend some time with your favorite Organization members. You can play them in missions too. I liked it well enough, it wasn't amazing but it didn't suck. But that's just how I felt, people feel different about different things. I would recommend it.
(Isn't this supposed to be at side stories section?)
Anyhow, the story is really good (tho it has nothing to do with Sora, it's more like behind the scenes). If you can afford it and have a DS, then it is a very good idea to go for it.
P.S: you might think about going illegal by downloading rom + emulators, but that ain't happening, nearly all DS roms are shut off now.
Its fun, i enjoyed it, it'll be good to tide you over until Birth By Sleep comes out. No it isn't really anything relevant, but you do find out alot more about what was happening in the Organization between COM and KH2
It's a fun game that's easy to get tired of.
The missions get repetitive after awhile. It can get really annoying and boring.
And it doesn't have much relevance to Sora's story. All he does is sleep. It's more about Roxas, Axel and Xion.