Sure. I like to divide humanity into myself and everybody else. It’s just as sensible as any other division, and much easier to keep track of.
If I’m feeling broad-minded I could include my brother, since he has pretty much the same mix of genetic information as I do. But most days I’d rather leave him out of the group. 
Walked into that, didn’t I?
If you do it with a priori reasoning like you are, I agree it would be fairly arbitrary, and difficult to defend. But statistical analysis of data is often used to throw up clusters defined by the data itself, based on similarity/dissimilarity of certain variables in the data. Assuming here that genetic information about populations is usable as such a criterion, a similar analysis could bring down(not do away with) the level of arbitrariness. You would then be left to define how many groups and by observing the level of within-group and between-group variance, it could be possible to arrive at a defensible number. Based on some of the (non-race oriented) responses I was reading about division into sub-populations, I’d figured that was what was being done. When I think about it some more, it actually seems quite unlikely that so much genetic data is available.
ETA:
was classic 