The thing is that at the hospital we have to test your blood each and every time you may get blood. While you will always remain A, B, O and Rh negative or Positive, over time- especially if you have had transfusions, you can develop other things such as an anti-D, etc- which are smaller components to blood. So knowing your blood type does not save you from a blood draw for typing and matching.

Now, if you are headed to another country, yes, know your type.
Or if you want to be a power donor for the red cross- of course…