The Boeing FCTM has this statement that the autothrottle ARM mode is normally not recommended because its function can be confusing.
Some features normally associated with the autothrottle, such as gust protection, are not provided during ARM mode.
One example how it can be confusing is when you sometimes get speed reversions on an approach - where the speed bug might “jumps” back up to a higher speed even when you are on target speed.
Many pilots use ARM and this is not prohibited.