It's logical that Nintendo would test the waters with LAN gaming before going to full online games. See how many BBAs they sell, see what the reviews of the LAN system say, see how the customers like the game...
The cool thing is that Nintendo seems to want to use online gaming to actually ADD something to the game, not just use the basic boring idea of "more levels and patches because, well, the game wasn't exactly finished" or "play online multiplayer because you got no friends!"
But something like Animal Crossing... or Mario Party... hmmm...
Sort of like the GBA-link cable thing. They had functionality to do a lot of cool things with games for a long time, but it wasn't until they started doing cool and new things that 3rd parties started to look at the device differently. Zelda has the tingle-tuner, and now SplinterCell has the downloadable map (which is a really cool idea). Now that Nintendo has shown 3rd parties that you can incorporate it into an option yet highly useful game tool, more companies will use it. Just like with the BBA and Mario Kart -- they chose a really popular game (more popular than Zelda) to really push the LAN capabilities -- a party game with 8 karts on the road, all of which controlled by people! -- and encourage 3rd parties to go the extra mile in developing that part of the code. TS2 would've had LAN on GC if it came after MKDD, that's for sure.
The only problem is that the other 2 consoles don't really encourage anything, so developers are just as likely to bail on a promising cube project in order to release a crappy ps2 or xbox game, because they didn't want to compete with Nintendo. I don't entirely blame them, but it does tend to lead to lower-quality games.