That Assassin's Creed III performance would worry me just as much or more. It's coming from the company that has offered the most third party support of the Wii U thus far. Of course, ZombiU did well, so who knows.
As far as attach rate, it should always be higher with a lower installed base, especially at launch.
I would wait until after Christmas before worrying about this, however. There is no telling how many games people are holding off on purchasing because they expect them as gifts. The system needs to mature a bit. The people who bought two or three games at launch need time to finish those and go back for more. Having more titles at launch means a lower attach rate on an individual basis.
Still, Nintendo is on tricky ground here. The industry is watching. If people want multi-plats then they're going to need to buy multi-plats. Not necessarily right now, but the Wii U is going to need to perform well on concurrent releases.
