That being said - self-driving cars will likely allow manual overrides or require a driver in the case of a manual override for the same reason space shuttles have a cockpit and give astronauts the ability to manual override the controls. People will want the ability to have control just in case.
If a system fails, or says it is failing, and you feel you know it will work or have an alternative that might work. Having manual overrides as a fail safe for software failure can be life saving.
For example, what if your brake sensor malfunctions and your car ceases to be able to operate it's brakes? A manual pedal to brake the car could save your life. Although arguably redundant it will likely remain as part of the car until enough time has passed that such fears are considered completely unfounded.
"There hasn't been a brake sensor failure in over 25 years." Such a statement could probably convince people a manual override brake system isn't needed and they can save money by ditching it.
A flooded (but passable) road is an example that comes to mind--I don't envision us programming our way out of that one.
Surely there's dozens of similar examples.