My sister and I both have (had) anemia. Mine was controllable, hers was not. She almost died at age 3 (and would have, if she had not had a blood transfusion). Yes, there are risks, but blood donation is (now) a very controlled industry and I think if there is a enough risk, it can mean life or death for some with severe anemia. Good luck!
My sister is now 20 and doing just fine.