World Of Warcraft: Patch Required for Burning Crusade

Nethaera tells us we should keep our 2.0.3 patch because:

As the Burning Crusade expansion was finished prior to the most current patch versions it will be necessary to re-patch to 2.0.3 once the expansion is installed. As some players remove their patches after they have been applied we would like to provide a helpful warning not to delete the "2.0.0-to-2.0.3" patch file as it will be required after the Burning Crusade is installed.