I'm not currently a primary teacher, but I have been. I also taught a course where this was one of our textbooks. UBD is based on the idea that you begin with the end in mind. You design backwards, by creating your objectives and assessment, then designing your lessons. It really is the only way to teach and assess.
Though I am not an elementary school teacher, I would tell you that UbD should be used by teachers to plan units of study at all levels. According to Authentic Education, "UbD works within the standards-driven curriculum to help teachers clarify learning goals, devise revealing assessments of student understanding, and craft effective and engaging learning activities." More importantly it helps students feel excited about education and it helps them feel like they are a part of the educational process as opposed to education being done to them. When teaching, especially at the upper levels, a teacher's practices should be overt. Explain to students how and why they are doing the activities they are doing. Engage them in assessment by having them help to create rubrics and then ask them to self-assess against the rubrics to set the standards with them for what is the stardard for each level of assessment. This also helps students feel that they can set goals for achievement and can monitor their progress. Take, for example, the skill of engaging in oral discourse about literature. If this is the end goal for the unit then a beginning would be for students to be presented with the rubric. The teacher would then show students how the skills are being broken down into steps and stages that will eventually lead to successful literary discourse (preparation of materials, listening skills, synthesis of ideas, etc), they are motivated to engage in the learning of those steps and stages so they can succeed in the overall skill. Of course, choosing a piece of literature that they will enjoy working with as the work through these steps helps too.