Brain death is essential to current practices of organ retrieval because it legitimates organ removal from bodies that continue to have circulation and respiration, thereby avoiding ischemic injury to the organs. The concept of brain death has long been recognized, however, to be plagued with serious inconsistencies and contradictions. Indeed, the concept fails to correspond to any coherent biological or philosophical understanding of death.
Robert Truog and Walter Robinson in "Critical Care Medicine"

As M. Potts and David Evans note in the "Journal of Medical Ethics":
There were never sound empirical grounds for criteria of death based on the loss of testable brain function while the body remains alive. One difficulty is the near impossibility of diagnosing—with the necessary certainty—the "irreversible cessation of all functions of the entire brain, including the brain stem" while the rest of the body remains alive.