It's only a positive if you have ample money to pay a very good lawyer to make sure that the data is interpreted fairly.
Here's the scary part:
Traditional techniques like measuring skid distances can be used to determine vehicle speed. Why would the EDR data be so different?
When I was younger and drive more cars with automatic transmissions and poor tires, it was not uncommon to get your 120 MPH speedo pinned while you're spinning your wheels, going nowhere. It's possible to be a driver on a slippery road, who's crawling along with your wheels spinning, and have an out-of-control driver at speed slam into you, killing a passenger. You're badly injured but the other driver walks away from the wreck, and proceeds to lie up a storm, telling a story that blames you.
Between the ice-covered road and the tracks made by the emergency vehicles, the physical evidence was destroyed. The cops should have preserved it, but to cover up their mistake they suggest using the EDR in your car (the other one doesn't have one). They see the 100MPH+ speed numbers and you're toast. You wake up in a prison hospital to learn that you've been convicted in absentia of voluntary manslaughter.
Think it can't happen? The real question is "would you bet your future on it?"
When an airliner crashes, the pilots get a lot of support from the pilot's union. In practically every crash, all other parties (the airlines, the plane manufacturer etc.) are reliant on each-other for business, but are more than happy to throw the pilot to the wolves as the scapegoat. The pilots have to fight hard for fair treatment. And all too often "pilot error" is stamped as the primary cause, even though every plane crash has a complex cause.
As a driver, you don't have a union to back you up, and because hundreds of millions of dollars aren't on the line and car crashes are far more frequent, you're not going to get the kind of attention to detail that plane crashes get. But interested parties like the auto maker would rather it's you who gets the blame. I wouldn't want to risk my freedom on such a slippery slope!
Bit of a stretch there, no? Despite the poor conditions and lack of crime scene preservation in your little story, there's a good chance a person with common sense could deduce what happened based on the damage to the vehicles.
Here's the way I see the "black boxes", if you're not doing anything wrong, what do you have to hide? Yes, you should review your insurance policy and make sure you haven't signed that they can use the data without further permission, but the cops and the court need a court order or your permission to review the data. However, some of us (myself included) have likely not physically signed anything to agree to ANYTHING different than what they originally agreed to when the policy was put in place (around 10 years and 2+ vehicles ago in my case).
Unless things have changed, the EDR will only maintain crash data if the airbags deploy. I was rear-ended last June, neither vehicle had airbag deployment (though his hood crumpled nicely). For all the cops know, I slammed on my brakes because the guy was riding too close (this was absolutely not the case, just making an example) but because there was no airbag deployment that data isn't recorded so it's a he-said-he-said situation.
For what it's worth, I'm on Nationwide, and even with employer discounts or other "promotional" discounts, other providers can't beat the reates I'm paying, based on the quick quotes on their sites. Under $1000 a year for full coverage on a brand new vehicle (well it's 18 months old now) and renter's insurance.