1) sometimes AIs are nice and dont want to steal, living you with agent recruits for the rest of the game
2) sometime you have so much money that benefit is insignificant
3) Needs experienced agents. Thus you stealing tech or having bad relations so other steal from you
4) valid use. Recruit can do this
5) valid use. But takes a lot of turns of giving introductions to switch diplpmats from nation to nations
6) very nice use. Its annoying that set up time is very long and that number of agents is very low through the whole game.
If you have a huge tech lead, and you have so much money that there's no point in rigging city-states, and you're bored with the lack of espionage options, then go up a difficulty level. If you're already at deity, then play multiplayer.