From a gameplay point-of-view, espionage is in bad need of an overhaul. It's just a woefully inadequate system that's unplugged from the rest of the game's mechanics and basic concepts of give-and-take. Since a civ can't do much with tech or sopols or other resources to become specialized at espionage, nobody can ever have a major leg-up on anyone else. It's a pointless tug-of-war where nobody can sustain an advantage in leverage.
It would suck to expend a great person just to have your benefit of that expenditure yanked away by someone's spy (costing them effectively nothing to do so, whether they succeed or fail). Then again, due to the aforementioned lameness of espionage, you likely as not can just steal it back 10 turns or so later. And then it gets stolen again, and you steal it back, and you steal somebody else's painting, and they steal it back, and on and on.