If Adam and Eve were genetically AO and BO , for example, their children could have had AB , AO , BO or OO genetic make-up, giving AB , A , B , or O blood types. If Adam and Eve had 56 children, as per Jewish tradition, then there would have been about 14 of each blood group. However, the claim of 'flawed design' is unscientific unless one can demonstrate a better design. And the first rhetorical question presumes that just because we are ignorant of a use, there can be no use. In fact, it is in principle impossible to prove that an organ is useless, because there is always the possibility that a use may be found in future.
Severe immunodeficiencies result when the thymus is rendered useless or destroyed.
This is the main problem with AIDS victims. Their T lymphocytes are destroyed by the virus. Deals with the appendix, 'fifth toe', wisdom teeth, male nipples, and many others. Even the appendix, widely thought to be a useless evolutionary vestige, contains lymphatic tissue and has a role in controlling bacteria entering the intestines. It is also a mistake to assume that there is no use for the blood types.