Another thing with spies is there is still the randomness of some outcomes. My third tier spy acting as a counter-intelligence agent in a city with all the anti-spy buildings and policies will still sometimes fail to stop an enemy spy from stealing my tech. This sort of thing can't happen with combat units. You will never see a spearman walk up to a dragoon and roll a D20 die, get the right number and dispatch that dragoon! Though there still may be an element of randomness in combat the outcomes are never all or nothing. Every way you look at the spying mechanic it looks too much like a late idea tack on by Firaxis rather than a fully formulated mechanic.