Not sure I can explain this well enough, but I'll give it a shot. Assuming Nintendo releases a Wii U Remote/Nunchuck:
1. The Wii U remote is NOT backwards compatible (cannot be used on the Wii) [edited for clarity].
2. Any Wii U game that uses Wii Remote Plus is also compatible with Wii U Remote.
3. The opposite is NOT true. Developers will not be required to (and most will not) consider a Wii Remote Plus mode. However, most Wii U titles will include both a tablet controller mode and a Wii U Remote/Nunchuck mode.
This is pretty much the same thing we saw on the Wii with Gameube controller compatibility. The new controllers are always favored over the old one because backwards compatibility is a bonus.
I've considered the pros and cons of this approach with a Wii U Remote/Nunchuck but I believe the pros far outweigh the cons. Including Wii Remote Plus compatibility allows Nintendo to continue offering a more simplified experience to casual gamers who are supposedly confused/terrified by anything but a large A button. They will never HAVE to purchase/use a Wii U remote as games made specifically for that segment of the market will never require more than the Wii Remote Plus and possibly the tablet controller for its screen.
Fair enough, but those same people will also never play or even bat an eye at Call of Duty, Zelda, Ninja Gaiden etc. Nintendo absolutely must make sure that the people who play those games are taken care of while also making sure that they never HAVE to buy Wii Remote Plus (some find it limiting). The Wii U Remote/Nunchuck allows Nintendo to offer a controller that doesn't insult core games/3rd parties while also offering the functionality Nintendo pushed for an entire generation. Personally, I believe there's a place for motion controls in core games... when used properly. If motion controls are an option rather than a requirement, developers are more likely to experiment and use it intelligently, on an as-needed basis.
Of course, this mostly hinges of whether multiple tablet controllers are possible. If they aren't, the Wii U Remote/Nunchuck is the best solution. It doubles as a Classic Controller and a motion controller. If they are, a Wii U Remote/Nunchuck might still be something to be considered since it can do different things than the tablet controller, due to being 2 separate pieces.