Put another way, if autonomous cars started off working on 0% of roads and you want them to eventually work on 100% of roads, well somewhere in between you have to pass through 1%, 5%, 10%, 25%, 50%, 75%, and 90%. It's rather disingenuous to criticize them for not getting all the way to 100% in one fell swoop.
So who decides which 50% (or whatever) of the road is suitable for auto control? My daily drive takes me through easy bits and much trickier bits, sometimes changing within 100 yards and back again, and how tricky depends on what other drivers are doing. What will the "50% competent" car do? Will it be saying
"Quick Dave, it's tricky, take over ! - It's Ok now Dave, let go the steering wheel - Oh hang on it's tricky again! - Now it's OK again - Oh jeez, some idiot's just pulled out in front I can't cope, LOOK OUT !! where the fuck are you Dave? TAKE THE FUCKING STEERING WHEEL DAVE !!! Aaarrrgghh !!! "
Sure you might get the stray deer hopping through traffic that requires a human to take control and improvise.
So the human must sit there with just as much attention, and with just as much skill as if he were driving anyway. So much for those hopes of drunks/non-drivers/blind people etc.