This is my perspective on it. The difference is 12x as much information about you. Now there is a single annual data point. Under this program you will be reporting monthly data points on your mileage.
Insurance is a business that makes a profit by charging premiums that exceed the probable payout of claims. Insurance is data and analysis of data. Let’s take the example of a 12,000 mile a year policy on a XLR. The insurance company thinks you are driving it 1000 miles a month with a corresponding risk. But, it sits in storage from November to April being driven a total of 500 miles in the 6 months. Then, you hit the road. May = 1500, June =2500, July =2500, August =2500, Sept = 2000, October =1000 Total = 12,500. The insurance company probably has an average for 12,000 miles policies where they give you a grace amount to 12,600 based on statistical data from all their 12,000 mile policies before flagging it for further analysis. Why tick off a good paying customer for being slightly over? They don’t want to lose your business. Now, they accumulate monthly data from Onstar instead of the single annual data point from their letter. The average they expect is 1000 with a max of 1300. Anything over 1300 gets flagged for further analysis. The summer months indicate that you are driving the car at a 30,000 mile a year rate versus the 12,000 mile policy that you are paying for. Next you’ll get a call from your friendly agent increasing your premium.
Interestingly I didn’t get a letter with the Onstar offer for my CTS.