Claims that nations are natural cultural units are also dubious. Many examples exist of nations with sharp internal cultural divisions including Malaysia, Lebanon, and Ethiopia.
Claims that nations preserve cultural distinctiveness are also dubious. Many nation-states make great efforts to erase internal cultural differences. Meanwhile, empires far from imposing a single set of universal standards often rely on or positively encourage cultural differentiation as a strategy of divide and rule, and to keep harmony between the various groups within the empire by ensuring none feel threatened by others imposing its values on the other.
Cultural distinctiveness need not be respected if we find its values morally objectionable. Equally, other things are more valuable than cultural distinctiveness such as peace and prosperity and these are better provided by large non-national units.