Usability testing is the most important measure of how people and products work together.
Raw power and numerous features are meaningless if a product is difficult to use. The true measure of productivity is not how fast a product performs a given task, but how quickly a user gets results. Usability testing adds the crucial third dimension to round out performance results and feature checklists. For users this means not only lost productivity, but also increased frustration that sometimes causes users to abandon your product and go to your competition. In the case of e-commerce, the result is an inability to complete transactions.