Personally, I think using roImageCanvas for the base and teaching super simple BrightScript techniques on top of that would be good for a tutorial. The canvas object has a really low barrier for starting to use it, and it's easy to have something that visual on the screen in 10-15 lines of code. Throw in a loop or two and some string manipulation to display stuff on the screen, and you can have a really quick tutorial that covers CreateObject(), arrays, functions, and event handling. All the examples are just more complex variations on that.
While I agree that a roImageCanvas tutorial would be worthwhile, I don't think I agree that it makes sense as a base tutorial. 95% of the channels out there don't use the image canvas at all, and I think most of the components in the SDK are designed to make it easy for a developer to write a channel quickly without having to worry about complex UI elements. Using the image canvas as a basic tutorial would probably be more misleading than helpful, IMHO.