Fine. If you choose to use the term race in an anthropological or cultural setting to indicate that, for example, people who have lighter skin had a better shot at social acceptance in the 20th century than people with darker skin, then having a less than rigid boundary is not going to invalidate the term.
The guy who went through the birth records of Virginia in the 1920s, reclassifying as colored anyone who had at least one great-grandparent who had been a slave, was successful in denying voting rights to various people based on “race.” The results of his actions were real, and however warped his criteria, they had a very real effect on people. In that case, “race” was certainly “real.”
When moving into the area of biology, however, when we find that there are no genetic criteria that can be applied to any large group to identify them, what are you going to use biologically to identify “race” and what would be the point in trying to do so?
In other words, we grant the use of the term in a cultural setting. Are you trying to insist that there is a biological reality in the face of the evidence? Or are you asking why we need to use real definitions or real facts in a biological setting?
Once more: Race can and has been used in cultural settings and I do not know anyone who claims that no culture can impose the idea of race on any group of people. (I know people who argue that the “racial” identification is counterproductive, but that is a different discussion.)
Race cannot be used in a biological setting because the genetic evidence indicates that it is quite possible for a Scandinavian and an Ibo to be closer, genetically, than an Ibo and a Masai–or then two Ibos.
From the perspective of biology, what does the idea of race buy us? We cannot show that any “race” is smarter, healthier, faster, better able to breed, more (or less) resistant to disease than any other “race.” So what is the point of lumping people together in a scientific discipline who have no scientifically discernible uniqueness?
Smaller populations do exist, with greater or lesser degrees of shared traits that are unique to that small group. None of those groups are large enough to be considered a “race” as the word has typically been used. (And applying the word to those small populations would simply confuse the issue.)
The reason to avoid using race in a biological sense is to avoid falling into the error of the authors of The Bell Curve, who thought they could discern a reality that did not exist (and who squirreled their numbers to make their point).
Do you have an example where the word race would have any application outside a cultural context?