It wouldn't really help to have your blood type on your drivers' license. If you need blood they are going to type and cross you before they release packed RBCs or whole blood even if you tell them your blood type. (You don't need crossmatch for some blood products though like fresh frozen plasma or cryoprecipitate, et cetera).

If you were going to have elective surgery that likely would cause you to need transfusion it would be good to know so if your type is rare they would have enough in the blood bank for you.

Also your blood type can actually change although that is rare. Change in blood type can be caused by certain infections, cancers, autoimmune disease or bone marrow/organ transplant.