Usability testing gains it's strength from direct user observation. Like listening to someone describe water vs. putting your hands under a tap to directly experience water, direct live observation of a usability test is more "real". Moreover, direct observation is critical for accelerating understanding and unifying decision-making. Live usability testing observation can not be underestimated. All too often stakeholders (and even clients who hire usability consultants) will skip the test, in favor of reading the report or watching the videos later.
Bottom line: You are wasting a learning opportunity (and your companies money) if you do not attend a usability test for a design of which you have stakeholder say in direction of a site or product.