For the most part, this decision will have both positive and negative impacts for consumers and developers, and depending on what Nintendo does will determine whether the decision is more positive or more negative ...
If Nintendo creates a well formed API that contains most of the functionality of a unified online service, makes some core features mandatory (unified login and friend list for example), while leaving most features optional, and removes many of the restrictions that exist in other online services they could have a "winner" here.
Essentially, while I'm not a big Facebook fan I recognize that a lot of people see value in social networking. Right now no online gaming service integrates with social networking in a meaningful way, and publishers are not really allowed to come up with their own deals with social networking sites to develop their own meaningful features; and it is likely that publishers will only get to implement the features Sony/Microsoft want them to if they create these integrations, and will mandate that they're included regardless of whether the publisher sees any value in it. In contrast, if EA worked out a deal with Facebook to make it possible for someone to earn "Bragging rights" on their Facebook page (or some other integration) they could implement it without waiting for Nintendo to see value in it and integrate it into their core API; of course, the side effect of this is that EA could secure exclusivity in features and prevent other publishers from having a similar service.
Ultimately, I would hope that Nintendo created a full suite of high quality and creative features for an online service (possibly partnering with many other companies) and are leaving it up to developers how to use them; and that Nintendo isn't just producing the bare minimum features and expecting developers to build complete systems themselves.