So, what's the deal? First, there is no actual microphone (a device capable of converting vibrational energy into electrical signals) found within the Wiimote. Though basic electrical engineering principals allow a standard speaker to act in such a fashion, the signal quality and frequency response would be unacceptably low. This chip actually requires an expansion device for it to be used at all. So why add it? It's only two dollars, but this rather unnecessary addition has cost the company several dozen million dollars. We can only assume that Nintendo or significant third parties are planning a title utilizing an expansion microphone, which will be presumably cheap enough to bundle with a title and incur no markup in price.
God knows the masses need a mainstream karaoke game (we know about Karaoke Revolution for the PS2, but it never really took off), and the Wii is just the system to do it. Like a Virginnnnn ... hey!