Some people consider that QA is an essential phase of any translation project.
Others simply assume that QA will not improve the project in any significant way. Indeed, one could argue that everyone involved in a project should perform their tasks to the highest quality standards, and therefore no QA should be required.
Many factors therefore need to be taken into account when deciding whether or not QA should be performed. After weighing the pros and cons for a specific project, one can then decide whether or not it is worthwhile performing one or several types of QA, and to which extent.
In any case, if QA is performed by competent staff, it will generally help to improve the quality of the project, or at least guarantee that quality has already been achieved, which is obviously also essential in any quality process.