Works for me
Antis do like their "If you've got nothing to hide then your've got nothing to fear" reference, don't they?
Problem is that in this case, the accident has already happened. People are dead/injured/shocked, and tens of thousands of pounds worth of metal is written off. Would you like your no-claims bonus to be stung for that? (Assuming you have fully-comp, that is - if you can only afford third-party or third-party-fire-and-theft then you're even more completely stuffed.) Or would you like a way of proving that you were driving safely and the other person was driving like a tit? And more seriously, would you like to be done for "causing death by dangerous driving" when the whole accident was caused by someone rear-ending you?
This is genuinely a case where the only losers are the people who commit the crime. If you don't mind dangerous drivers getting their insurance claim, buying another car and doing the same thing the next day, then fine. Me, I'd like the black box, please.
I note people raising the prospect of auto-reporting to some central database. This ain't gonna happen, mainly because it's technically implausible. You'd need to invent a standard way for cars to talk to base, and this doesn't exist yet. And roll-out for tech like this is on the order of decades, and would only happen if the US was interested - the UK market isn't big enough for this to be worthwhile. Not only that, but you'd also need reliable enough measurement of speed to be legally valid, and that simply isn't possible in a car because wheelspin happens.
In the case of a black box, the wheelspeed will be a useful guide. But most critical is the accelerometer data when the impact occurs, which tells the investigator the relative positions of both cars and the speed of impact. Brake pedal sensors are the other useful factor, saying how soon before the accident they braked (if at all). Wheelspeeds will be useful too for capturing events like spins or loss of traction - if you can prove that you spun on ice, you're likely in a better position than if you just drove clean into the back of someone without braking.