They work better together because the DS can get game demos from the Nintendo Channel on the Wii.
The DS is a game for the far ends of the age spectrum– adults and younger kids, although there are plenty of in between games, too.
The built in application, Picto-chat is a missed opportunity. You can’t chat beyond 30 feet, you have to communicate with the other person by a separate channel before starting a conversation, if the other user is playing a game, you can’t signal that you’d like to chat (unless you walk over and tap them on the shoulder). If you are that close to each other, why not just talk? The only use I can think of for this is so that kids can communicate when situations dictate that they must not, e.g. in church pews, during lecture or when the parents are willing to put up with the noise.
As for adult games, I got the My Word Coach, My French Coach and My Spanish Coach. Several people tried out the games for a non-trivial amount of time, so they were unusually successful for essentially flashcard software. I want to get left-brain/right-brain to train my self to stop using my right hand so much, on account of my repetitive strain injury. I will probably order Brain Age next.
Internet was another missed opportunity. Setting up wireless is too hard. 5 year olds can’t be expected to type in a SSID. If an app was going to be built in, it should have been the Lynx text browser. Trying to implement Opera, a graphical browser on a DS was a fit of megalomania on Nintendo’s part.