There is always something to work on, no matter what level the student is, no matter what level the material was originally introduced. We work on basics every class - more senior students may be working on a different aspect of the basic techniques than the junior students, but they all work on it together.
First, if you lose sight of the basics, the more advanced techniques won't work - because in the end, everything relies on those basic techniques taught to junior color belts. If you don't train a technique, it becomes rusty - and therefore anything based on it won't work as well or be as well understood as it should be.
Second, once past a certain rank, most students are expected to teach. That means you need to understand something thoroughly and perform it correctly - or you won't be able to explain it, instruct someone in its proper performance, or demonstrate it properly.
Third, I don't know about your students - but mine tend to practice the things they enjoy the most when they are away from class, unless there's some reason (like an upcoming testing) to practice something else - so they get better at things they like, and ignore everything else unless forced to do it - which brings us back full circle to rusty basic technique affecting the quality of everything else.
