Imagine you are surveying a concrete wall in a round tank in less than a meter of visibility. You are looking for cracks. Your only positional reference is the curvature of the wall and ROV heading.
The tether is pulling you off location because you had to go through columns to get to this section.
While trying to fly up and down you knock your heading off and you drift left.
You are surveying a jacket. Each face has known coordinates. Visibility is poor, current is present and pushing the ROV around, once again you are relying on heading and structure for positioning. You need to do an anode count survey.
While fighting against current you accidentally change your heading. You are now inside the jacket and completely confused.
I could go on with different scenarios.
It is very easy to change your heading by accident. This happens all the time. Especially if you are using a 3-axis joystick. (Using an x-box controller is not suited for commercial applications.)
The auto heading during stabilize and depth hold works great in most scenarios and I would not want to change anything.
But job sites are not very accommodating. You have to dive in all kinds of environment. Poor visibility, current, difficult structures are some of the difficulties we have to deal with. Acoustic positioning is not always an option. (In most of my work with the BR2 it actually seldom is) The ability to lock the heading would help a lot.