Major Description
A use-case: Recently, we stumbled on a bug that wanted us to disable the feature to run a debug script in map/reduce on tasks that fail, specified by mapred.
{map|reduce}.task.debug.script. The best way of disabling it seemed to be to set it with no value in the cluster configuration and mark it final. This did not work, however, because the code in configuration explicitly checks if the value is not null before adding it to the list of final parameters.
Without an ability to do this, there might be a need to explicitly have a special way of disabling optional features.