All of the above are really good reasons to lock it. I started out using auto trac and even though I was still getting used to my MXT, same feature, I noticed that I would come across a deep signal and when I stopped to check it out it was never as good as the first swing across it and usually dissapeared. I moved on assuming they were flukes or just blaming the machine for being noisy, ect. Eventually, I messed with the auto trac and the lock feature and the next few faint signals that dissapeared I stepped back and locked, then GB'd the machine and went back over the faint signal and it was there with every swing and didn't fade away.
The other motive for locking it has also been stated, trashy areas will not give the machine an accurate area on the ground to 'maintain' a good ground so it will drift all around and have no idea if it is balanced or not. In trashy areas you want to find a clear spot to 'ground grab' while in 'lock' and then procede to detect the trashy area without the machine changing its ground balance.