While no system needs achievements, they're something a lot of people enjoy, so it's surprising they keep ignoring them. Done properly, an achievement system can be quite awesome. Devs get a bit of free publicity from having their game show up in a gamer's friend's recently played list...gamers get incentives to play the game differently than they might otherwise play it, or an entire new meta-game altogether.
For me, it's more of a "why not?" scenario when it comes to implementing achievements.
On another note, I really hate the term "Wii 2". Something about it just irritates me. Nintendo's never been one to simply slap a 2 on the end of product to signify the next generation. If anything, i'd prefer the term "Wii Successor". Doesn't roll of the tongue, but it's much more accurate.