dtgreene: Well, there is the fact that you need Cartography to actually get the game to self-map, and if you erase that skill, the game will not map for you. (Sorcerers start with it.)
Also, note that there are class quests, most of which you won't have your Sorcerer with you, so you won't get the auto-map unless you taught the skill to someone else.
Furthermore, the auto-map doesn't show placed encounters or events. (Knowing where placed encounters are can be handy since it will let you Jump (S 2-4) over the encounters.
So yes, there are legitimate reasons you might want to make your own maps.
Side note: You might consider checking out Elminage Gothic, a Wizardry-style dungeon crawler whose automap requires using a consumable (or spell) to display. If you enjoy making your own map, you might find that aspect of the game fun.
The "Etrian Odyssey" games for DS/3DS also allow you to make your own maps, while by default the map automatically draws itself when you move, you can turn that feature off and just map it completely from scratch.