Lacking a clear notion of who the users are and what they do, how could one even know what questions to ask or what to test in the usability lab? In science jargon, asking irrelevant or just plain wrong research questions is called 'Type III Error'.
The common practice of conducting usability testing prior to rigorous Ethnographic study, such as Field Observation and Depth Interviews, is a principal reason for so much usability testing being a waste of time and money.
By analogy, conducting a usability test before Personas and Scenarios are developed is like booking a recording studio before any of the songs are written--very expensive and very little valuable output.
This is why it is essential that Usability Evaluation Planning be done as an integral part of Researching the Opportunity and take full advantage of the insights from Ethnography, Initial Market Research, Human Factors and Persona/Scenario Refinement.
