@kkeith29: Just because you can't think of a reason, that does not mean, that there cannot be a reason for this.
Right now i have 2 Joomla-Installations: cms-dev and cms-prod are the two folders. Now i want to copy cms-prod to cmd-dev, but before that i need to copy the database.
So i include cms-prod/configuration.php and setup $config-live = new JConfig(), to be able to access the config-values for db-host etc.
Now i want to include the cms-dev configuration, to write the dumped mysql-data into the dev-table. But here php says: "Fatal error: Cannot redeclare class JConfig in cms-dev/configuration.php on line x".
The first thing i thought about was un-including the first configuration-file.. but i guess this is not possible and i have to create another workaround.
This was just an example. There can be many others, why one could want to un-include something :-p