Just wanted to make sure I understood the question, SD & rest. Boeing is carefully saying that it is normally not recommended because its function can be confusing. Modes may automatically revert and make the bug jump to a higher speed during approaches without A/P. The primary feature of the autothrottle ARM mode is minimum speed protection in the event the airplane slows to minimum maneuvering speed. This recommendation also serves as a disclaimer since other features normally associated with the autothrottle, such as gust protection, are not provided. Many pilots and instructors are use arm mode during approaches.