I don't think it's a smartass move to use spies to destroy defenses the round before an invasion - I think it's one of the things that a highly espionage based strategy can accomplish and there are sacrifices to be made to use such a strategy. Also...doesn't always work because sometimes you might find some cities are almost impossible to get spies into for a round+. That the AI doesn't use it or consider it much is just another AI deficiency.
I think the point *was* that "AI doesn't do it", no so much that "it's a great tactical abuse of the game rules".
I mean, Merging Units is a good example of how the human player can use a feature much more flexibly than the AI will ever do.
Namely: Merge a unit to the max, grant it a General, then SPLIT it into several lesser units that now have a significant XP boost, *as if* each of them was granted a General.
Basically, that's an example of "cheating" in regards to Generals - and while it's 100% accessible to the AI as well, it will NEVER do anything like that, because it has no idea it CAN be done.
Or boosting "focused growth" units by granting Generals to OTHER units in the same tile, basically "splitting the boost by half between them".
I'm 101% sure that the AI would only ever do this by a sheer pure coincidence and NEVER as a real tactic - very much unlike the human player doing it absolutely deliberately.
Would that be "cheating", though?