Kolechians themselves aren't useful. They're vile creatures that deserve to die. But we mine as well kill some of the more obedient by letting them live and then attone for their sins against glorious country by dying for it. Plus it saves other honorable Arstotzkans from dying.
Only problem is that dirty Kolech scum might try to betray glorious country and spy on it for Kolechia's purposes. Which is a massive problem, but they need to be reconditioned.