269 disengages over their entire fleet per year and perhaps 13 hypothetical accidents of unknown magnitude (but some quite minor: "2 of these involved simulated contact with traffic cones") if they hadn't disengaged is 'staggering'?
282 failures of their system across 434,000 miles is 1,539 miles between incidents. That's a system that fails more often than even old school cars need to change their oil.
Suffice to say, Google's claim that they're ready to remove the steering wheel and it's only the law holding them up, which is the argument they made on the floor of Congress... they're liars.
EDIT: Since I'm rate limited, my response to Retric below: False. The system disengages when the system has an error and has conflicting data about the state of the world around it. It isn't overabundance of caution, it's overabundance of confusion.
I don't think it should. A disengage is simply a precaution. Whether it would have been an accident if the self-driving car had continued on is a question of counterfactuals, and they examine it with the recorded data and simulations of the car software to see what would happen; and they report that of the 269 disengages, only 13 probably would've led to an accident with physical contact with something it shouldn't've, and at least 2 of those are trivial. You claim to have read the report in detail, so I'm not sure why I have to explain this.
PS: If they just wanted to add miles they would have operated on highways not the far more accident prone city streets.