The variance for most traits is higher within races than it is between them. I assume OPs point is the division into categories "black, white, asian, latino" is arbitrary, and you could easily reduce or add more groups as you see fit.
Theres nowhere in science you can look that will tell you how many groups there are and where to draw the lines between them - i.e a construct.
I agree that the pseudo races that you mentioned are quite arbitrary, but I suppose they could be useful terms in a population as the diverse the US. In less mixed contexts such as Ethiopia nobody is "Black" rather they would be called one of the 300 some odd races they have there. Likewise in China - nobody's "Asian" in China, rather they are Han or something.
There's probably a whole Venn diagram or tree to describe different races, how far up the tree we go to describe someone pretty much depends on how far away we are from them on that tree. And then there are people of mixed race - we are all human after all.
From an also non-USian.