Dale Earnhardt Jr should not retire just yet, the past experience in racing is something that can happen to any driver and that should not be a reason for him to quit. Let him come out at Indianapolis as strong as never before and make the NASCAR team proud again.
Earnhardt Jr. lost his father during a NASCAR race, one which was televised. He is certainly aware of the danger. His father is a legend and Earnhardt Jr. is one in his own right, as well. He has been quite vocal about his love for the sport and his actions speak even louder.
A concusion is a serious injury, especially when the symptoms are as long-lasting as Dale's, but only time can tell if it will correct itself. He has had two serious concusions, and that is many fewer than other athletes, such as football players, who are still active in the game and face many more possibilities for this type of injury.
As long as Dale Earnhardt can physically drive well, there is no reason why he should retire. If he is capable enough to drive safely, there is no reason to stop him. Not only that, but the number and severity of crashes has greatly reduced since the crash that tragically killed his father.