Teaching people to think of a society as a collection of “groups” is very useful, because then these groups can be pitted against one another for political gain. Gay vs. straight, white vs. black, asian vs. hispanic, old vs. young, etc… ad infinitum. Every minute in school spent teaching this sort of thing is a minute not spent teaching math, science, and language skills. This is also very useful – at least if you want a dumbed-down, easily-manipulated population like we have in my home country of Kazakhstan. Jagshemash!