Leave it to you guys to find it as a negative for Nintendo allowing users of the app the ability to add friends by the 2 most popular social apps in the world! I don't get the logic here. Nintendo isn't handcuffing anyone in this situation, most people aren't real friends with people they follow on Twitter. People who have demonstrated themail app already state that it's not monitored, censored or anything like that and users can do or say what they feel. So once again, how does Nintendo get such negative comments for a brand new social app that has every chance to grow and update it's features. Oh yeah and it's free, so let's complain about that now to.
As far as I'm aware you actually can add friends manually just by using their Nintendo ID. The twitter and facebook tthing is just more convinient and automatic for most people. You get automatic notifications if you happen to be facebooks friends or mutual Twitter followers, but you can still use Nintendo ID.