Here's the thing...Nintendo is totally separating their market with this add-on. There are certain things that you can do with the MotionPlus but cannot do without it. An early example is the drums in Wii music. I forget exactly what, but there was a certain move that the guy did that is not possible with a regular Wii remote (I read it in some preview of Motion Plus on either 1up or IGN).
I'm sure a bunch of people will have one, since it's being packaged with Wii Sports 2, but it shows a lot of arrogance on Nintendo's side. They just kind of assume that everyone is going to buy it. It's an even bigger pain in the ass for third parties. If they want to use a mechanic that's only available with motion plus, they're going to have to figure out how to get around it for people that don't want to buy one. It's worse than the 64 RAM upgrade, since this is actually limiting the controller and the gameplay, as opposed to the graphics. I'm really just kind of fed up with everything Nintendo. I said the same thing around this time about the Gamecube and vowed to steer clear of Nintendo this generation. I got caught up in the hype during E3 2006 though and bought a Wii at launch. They're going to have some serious explaining to do for me to support them next generation.