[TheCoreh]

It's qualitative research: as long as you take sufficient precaution picking the subjects to avoid obvious blind spots, it should be enough to get a rough idea of glaring mistakes or problems and to get a good feel of how users perceive and navigate the UI. It also allows you to individually observe effects that would often be "averaged out" by a larger sample. It's being used as a first step (before they conduct a broader quantitative research) so it sounds pretty valid.