I'm not into Voice chat with crappy quality. In general I don't like voice chatting with strangers. Guess what device in my house isn't used at all. My Phone (well maybe for music but thats not the point).
I feel more comfortable with a software that allows me to type messages. No accents to worry about or mispronunciations. The only thing to worry about are typos or, in case of Letter box, bad handwriting.
Also not everyone owns Pokemon/Animal Crossing and/or uses it purely for chatting purposes. Not to mention, not everybody has these games.
Nintendo Letter Box was a Freeware software that everybody had access to and allowed writing, drawing, sending pix and sending small sound files. For me it was a must have software on a 3DS and it was even pre-installed on newer 3DS Models.
I wonder how confused new customers will be when they try to use it.