Over the years, the JWs have offered four separate rationales for the allowance of various blood components
(1) In September of 1958 IgG preparations such as the diphtheria antitoxin were allowed on the basis that these "do not nourish the body." However since no component of blood actually nourishes the body, this rationale if taken to its logical conclusion would have brought the whole doctrine crashing down.
(2) In June of 1982, the 1958 rationale for the allowance of fractions was replaced. The JW's attempted to divide blood into "major" and "minor" components, with the major ones forbidden and the minor ones allowed. However the policy the JW's enforced did not consistently follow this rationale. The only basis the JW's ever offered for the major/minor division was raw percentage of blood volume for each respective component. Platelets which comprises roughly 2/10ths of 1 percent of your blood volume were forbidden while albumin which comprises slightly more than 10 times as much (2.2%) was allowed. Not one of the components of plasma was forbidden yet plasma as such (these components suspended in water) were forbidden.
(3) In June of 1990 the 1982 rationale was replaced. Blood components were now divided on the basis of transference across the placental barrier during pregnancy. The same divisions remained. This (IMHO) was one of their better and more honest attempts at interpretation as it put God back into the equation via an appeal to natural consequence. The idea was that God would not violate his own "laws" via creation. However this rationale was not technically viable either.
In 1992 a female lab tech who had donated a blood specimen for analysis was found to have "Y" DNA circulating in her blood stream. Researchers were puzzled until it was disclosed that she was 6 weeks pregnant. The source of the "Y" DNA was her unborn son. Cells in the blood of the fetus including fetal nucleated red blood cells, were crossing the placental barrier. Other studies detected fetal erythroblasts, trophoblasts, granulocytes and lymphocytes in the maternal blood. Since then it has been demonstrated that a woman can still have fetal blood cells in her blood stream more than 30 years after her last pregnancy.
(4) In June of 2000, the 1990 rationale for the allowance of some blood components was replaced. Blood components are now classified as either "primary" or "secondary." This enlarged the scope of permitted preparations procedures to any and all "fractions" or a "primary" component.
While I would agree that easing the restriction was a positive development, this is still a human interpretation subject to error, future revisions and lacking anything even remotely resembling a biblical basis.