Matthew Turland came across someone having an issue running his PHP-based IRC bot (Phergie) an an environment where the exec function wasn't allowed:

This causes a warning in the Quit plugin, which uses exec to automatically detection of the full path to the PHP CLI binary on non-Windows systems that it will later use that path to initiate a new PHP CLI process to "restart" the bot.

It check this setting for the future, it was recommended that he look at the SPL ReflectionFunction class (a part of the Standard PHP Library) that would let him check the disabled status of any PHP function (looking at the result of the isDisabled call).