My view is that having different levels of explanation is an excellent idea. One could even imagine having essentially the same article written in two different styles. To some extent we can obviate the need for this by using the hidden-text facilities, but that may not always be the best solution. For instance, Henry Wilton has written a very nice article on groups acting on topological spaces. He assumes you know what it means for a group to act properly discontinuously. Now one could just add some hidden text that explains that definition for people who don't know it (and in fact I'll probably do this), but one could also imagine an alternative version of the same article that started with an explanation of some of the concepts (freely discontinuous actions is not the only moderately advanced definition he assumes) and then proceeded with the article much as it is. Experts might not want to wade through an introduction of this kind, so it could be good to have two versions of the same article.

But I think we should be cautious about this: one can imagine other possibilities (such as an entire hidden section of text) that would probably work in the case of HW's article. For the quadratic-equations one, there is probably more of a case for writing an entirely separate article. Incidentally, my target audience for that article was somebody who had been taught about quadratic equations at school but had been left with an incomplete understanding of how all the various methods fit together, and of the fact that different methods are appropriate for different problems. It wasn't aimed at the genuine beginner. In principle, I have nothing against parts of the Tricki being extremely elementary, however. But if we start writing articles aimed at a school audience, then we may wish to label them clearly as such, and make navigating about in the elementary articles an easy process.