This is true: summary description is not always necessary in good expository writing. As a matter of fact, it all too often drags students away from the point of their essays, as it's easier to write summaries of what you've read than it is to find and clearly express your own insights--which is the point of your writing an essay in the first place.
The point of expository writing is to explain, analyze, or teach. Summary description may be necessary in some cases, such as when your intended audience is unfamiliar with the material you're covering. The need for it is also incumbent upon they type of expository writing you're doing. For example, process writing (in which you're describing and explaining a process) or a comparison essay wouldn't necessarily need a summary description.