I've said it before: the reason I want to see Nintendo consoles online is because the prospect of Nintendo games with online features excites me far beyond your standard FPS/racing/sports fare.
Mario Kart, despite its failings, was still a lot of fun online. Same with Animal Crossing. Now, if we could get an online Battalion Wars, Fire Emblem, Wario Ware, SSBB, Mario Strikers, Pikmin vs. mode, etc., I think it's safe to say these games would be absolutely awesome.
The real reason to have an online mode in these games is to allow players to ALWAYS have some competent human opponents, even when their friends aren't around or they kick the piss out of their friends at that particular game. Online modes increase the replay value drastically, THAT'S the real reason to include an online mode: it's the biggest favor you can do your customers and if it uses friend codes, it doesn't cost much in the way of online services to do.
How many Wii games are sitting on your shelf right now which you'd be playing on a regular basis if you could ALWAYS have a skilled opponent to compete against?
How many GAMECUBE games are sitting on your shelf under the same pretense?