This paper presents two case studies of prototype evaluation as a tool for user needs elicitation for emerging technologies. In the first user evaluation, a high-fidelity virtual reality prototype is used. In the second one, a low-fidelity mixed reality prototype is used. Our results show that prototypes may be a powerful a tool for eliciting user-needs, but their potentiality depends on their fidelity. In our studies, users elicit more needs when working with the high-fidelity prototypes. Furthermore, the information collected in this case is richer and more useful for design. We discuss these results as well as some factors that could help stakeholders elicit a greater number of needs for emerging technologies.