Usability testing is a powerful tool, and I feel it should be completely integrated into the entire product development process. There are many different methods of testing, such as a-b testing, focus groups, lab studies, eye tracking, etc. At the end day you need to learn how to interoperate it.
The problem is that it isn’t as scientific as people seem to think it is; it is in fact extremely subjective. You will learn allot about user behaviors, however the result is always subject to interpretation. You may be able figure out what a user is doing, however there is no real scientific context as to the why.
Often times a usability becomes a sort of Pseudo Psychology that ignores things such as intuition, consistency, context, and user training. Without this overall view of how every little piece of the puzzle ties itself together, often times taking usability testing literally can actually make things more confusing and/or awkward.
Designing interfaces solely by usability testing will create products that may be logically straightforward, however still confusing and flawed from the end user’s perspective. Usability Testing should be used as a tool to refine as oppose to a tool to Design.