Implementation isn't the only cost of features. You need to test them so they don't break. Features can be a burden for the codebase and removal can lead to many edge cases becoming unnecessary. A magnitude of features can overwhelm users, especially occasional users. They steal attention from other features that you may find more important.
And the most important point here: This feature really is broken as of today. Nobody can persuade me that they use it on their main browsers. I don't believe them.
But you don't need to disable JS altogether to prevent that, only disable replacing context menus. Unfortunately for that argument, updating to FF23 removes all of the 'advanced' JS options, not just the checkbox to disable it altogether.