Wii Play is just some $10 game you pick up because, what the hell, we're all paying too much for Wiimotes and nunchucks anyway. As Opposable Thumbs points out, it's a strategy that's paid off well for Nintendo. Because while not many people talk about group sessions of Wii Play at senior centers, it's been in the top five selling games for the last eight months straight.
In a way, Nintendo has succeeded off a form of supersizing. Don't sell the consumer two Wiimotes, just sell them more with one Wiimote. It's a small upsell that can clearly translate to big dollars.
Nintendo's secret best-seller: Wii Play [opposable thumbs]