Wii Motion Plus cannot become the standard. There are too many people who are light players and will not enjoy the frustration of buying a game and finding out it doesn't work when they get home. This will turn people off of the Wii (and probably consoles in general).
I don't see a new peripheral being any sort of a marketing problem with a pack in. Look at how much Rock Band costs for the ability to play one game basically. I could see other Wii Motion Plus specific games if it sells really really well, but this would require a special sticker or designator (a la the upcoming red WiFi icon).
Gee, you think about it, console gaming is becoming much more complicated due to all these options. I wonder if there will be an eventual backlash?