You are right that the riving knife will reduce the injuries most of the time but that is not the point. The point is to make the safest saw possible and if the worst case scenario occurs then reduce the injury to a scratch rather than lost fingers or a mangled hand.
Similar to saying: I don't think I need an airbag since most of the time the seatbelt by itself (if worn) (the riving knife is removable) will save my life or reduce my injuries 100% of the time. We know that is not true.
It is obvious that SS did not set out just to build a saw with only its safety technology. They set out to use all the safety features found in saws today and add their technology as well. It is by far the safest cabinet saw on the market. If you do some searching, it is well known that kickback can cause your hand to move into the saw blade. Will the riving knife reduce the chance of this occuring. Absolutely, but will it stop it 100% of the time, no. So in that case the brake does its job. This is just the same in a car. The airbag is a complement to the seat belt. One without the other is less safe no matter how you look at it.
Take a look at the testimonials on SS's website. Proof that with or without the riving knife the worst case scenario does occur and the SS kept the injury from being serious. Just a bandaid is needed in most cases.
http://www.sawstop.com/why-sawstop-testimonials.htm