Actually CA borders are pretty well drawn, as virtual absence of cross-border guerilla today seem to prove (all militancy largely directed against either "newcomer" ethnic groups or flares between clans of the same ethnic origin). They are extremely complicated, reflecting complicated character of settlement, but Stalin did not settle Uzbeks or Sarts where they lived, he just draw borders reflecting situation on the ground. Borders could be even completely arbitrary along the lines of ninebucks's idea, reflecting convenience of colonial administration (in the Somali fashion, and look how much good it did in Africa, with scourge of tribalism eating continent's guts for last half-century) or even more complicated, as Stalin built new "nations" out of related ethnic and linguistic groups, which otherwise could demand separate "national homes". Imagine 3 or 4 statelets occupying today's Uzbekistan, 3 Kazakhstans (Senior, Middle, Junior Hordes) and so on, 2 or 3 Tajikistans (although, in all fairness, this country today is a federation of two largely independent and hostile parts).