I have purchased several of these over the past 3-4 years, for my dogs and for many of my clients dogs.
A few years ago I got a call (Pet Taxi) that a car had been in an accident and the two small dog passengers had been thrown into the dash board and window. This happened because they had no safety restraints. This is only one of many accidents that could have been avoided. From then on I have recommended Seat belt connectors to be used with a dogs harness.