I don't know if this is a bug or just an undocumented design feature. But, as far as I can see, espionage is useless. Theoretically, with the highest level of espionage you're supposed to "steal" a technology every so often. But, over several weeks of play, I think I've gotten one or two technologies that way. For instance, in the last game I played, I pumped huge amounts of money into spying on the Torians (who usually out research me -- and did so in that game). I achieved that top espionage level really early. I never "stole" any technology from them. It would be nice if the game gave some kind of visibility in the "stealing" process. Perhaps, at intervals it could pop up a screen saying I've snuck around enough and have the opportunity to steal a technology: which one do I want?
Peripherally related to that is the "conquest" of a planet by defection instead of by invasion. If a planet defects to me, I'd say I should also get a technology just like I would if I invaded.