I could care less about the privacy, I get upset that they record the data that is easy to get rather than data that is actually related to safe driving. Sure braking force and cornering acceleration are things an accelerometer can easily and reliably measure data, but they have nothing to do with safe driving... These things don't record the things that are dangerous such as not using headlights and blinkers, weaving through traffic, tailgating, running red lights, driving down the wrong side of the road, eating/texting/having sex while driving, having a car with bald tires, and all of the other general asshatery that we all see and isn't private, but are things that are not factored in to insurance.
When insurance gives me a discount for having a manual transmission (know how to drive, have to anticipate actions farther in advance, extra theft protection), having high performance tires (greater grip, shorter stopping distance), having extra safety items in my car (roll bar, first aid kit, fire extinguisher), or having extra driving education (sure an online 20 minute "defensive driving" course can get you a discount, but competition track license doesn't). Then maybe I will consider letting them track my driving habits to reduce my insurance costs.
Until then, they charge us all based on the biggest idiots of the group and then offer discounts for things that make them more money: bundling (which doesn't reduce risk), tracking (which they can sell the data for), and longevity (loyal customers).