There was an interview in EGM that discussed this issue, though I think the example Lizard Dude offered is more than enough not to include it. Regardless, here's an excerpt from EGM's interview with Reggie.
EGM: A big part of the online experience is communication, but so far the online DS games have had limited or nonexistent chat functions. Why not enable something like PictoChat or voice? Is that coming to future games?
RF: Two things: First, we did do a tech demo of [online voice chat for the DS already], so it certainly is something we're exploring. But having said that, we believe what is most important to the [online] gamer is free [and easy service]. [That's what we believe is] going to get more than the current 10 percent of people who play online games. As we get some experience, if we find that the chatting aspect is more important, I wouldn't be surprised to see it show up in our games. But we have really focused [on] what our data says are the key drivers for a great online experience.
EGM: So, you're not closing it off, but right now you're not convinced that it needs that?
RF: Correct.