This is a big one, on par with the Diverse Empowered Team principle. Sponsor users are key members of that diverse empowered team. It is critical for leadership to recognize and empower these people to contribute to the design. As engineers we face a key design challenge in that we are NOT our users so our understanding of the use case is by definition flawed. Identifying sponsor users, inviting them into the design process, and adapting to their feedback is how design thinking helps us to mitigate this challenge. Engaging with sponsors can slow down design work, finding the correct sponsors and managing schedules so that we can collect and incorporate their feedback can take time, but it should not be overlooked. If you are doing time accounting, consider the time needed to engage with sponsors as a risk mitigation. These same sponsors will be using the product when it goes live, and finding and fixing problems in design is orders of magnitude less expensive that fixing them after code goes into production. Your engineers are always wanting more time to work on refactoring technical debt out of the system, give them some time.
It is difficult to overstate how important diversity in sponsorship is. A variety across age, gender, sexual-orientation, life-experience, work-background, culture, even political affiliation are all way more important than getting "computer savvy" sponsors. When building sponsorship cohorts, I like to include diversity across a variety of other spectrums that include Sr. and Jr. level staff, Technophile and Technophobe personalities, By The Book and Wild Ducks working styles.