DefaultKeyBinding.dict: moveWordBackward: is not overridden on macOS Sierra
I finally solved this after realizing that the difference between my El Capitan (10.11) machine and Sierra (10.12) was the new dead key on Option-b. Whether this is a difference between keyboard layouts or OS versions I haven't been able to determine.

It seems that KeyBindings are unable to override dead keys. Using Ukelele I was able to change the output of option-b to some other character. I then installed this keyboard layout and moveWordBackward: began working with no further modifications.


In Ukelele, Choose File->New From Current Input Source
Right click on the Layout Name and choose Set Layout name and Script


Rename to something like U.S. No B Dead Key

Double click on the layout name to open the layout editor.
Hold option, then right click the b key; choose Make Output
Enter any other character; it will be overridden by DefaultKeyBinding.dict
Save
Choose File-> Install For Current User
In the Keyboard Preferences, choose your new keyboard layout.