Sure, there could be some optimizations for reducing damage in an imminent crash scenario. That's just fine tuning. Google's real goal is to get a machine driving a little bit better than the average human. It's looking like, at least in known, well mapped cities, they have achieved that.
As for letting off the brakes when getting rear-ended, that may not be a good idea - the guy in front of you may not appreciate turning a 2 car wreck into a 3 car wreck. Especially if said impact pushed them into crossing traffic.
There's some room for "can I dash out of the way" thinking, but in general that's probably a bad idea too. Being predictable to other cars is better than flitting around erratically, possibly causing other accidents.