I find that all 3 agents types are useful and use them on all total war games I've played. The Assassin's role is the pretty much the same in both MTW and RTW; killing other agents and characters/generals. Spies were slighty less effective in RTW cuz they don't increase public order like they do in MTW but still usefull cuz they have huge "line-of-sight" for exploring and a high level spy can cause serious unrest in a rival's settlement.
Now Diplomats, on the other hand, I find very useful in RTW. If you're you're a filthy rich kingdom with lots and lots of cash to burn then your diplomat can be a deadly weapon. I've bribed away countless stacks of armies that I would otherwise have problems dealing with cuz my armies were elsewhere fighting. Also, who said that money can't buy friendship?If you offer huge sums of money you can often secure alliances you normally wouldn't get. I also found it very usefull to fund other kingdoms who are enemies of my enemies. That way my opponent will be distracted trying to subdue the other kingdoms while I pummel/bribe the rest of his armies to dust. I also rescued a number of factions on the brink of oblivian by giving them hordes of cash this way and most of the time they end up with decent a decent sized kingdom and armies to annoy my enemies after that.
Just watch out though cuz they might turn on you if they become too powerful lol.
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