Only minus to that tactic is that you need to rebuild the city once you conquer it.

Better idea would be to just use those spies to set off riots in and sow discord in faraway cities of Seleuceia and the poor bastards will send some forces to stow those rebellions effectively giving you a big break. Not to mention, some cities tend to riot to for example Pahlava and Sa'byn, which will further add to the chaos.

The whole burn-down-tactic worked a lot better in regular RTW, where you didn't have so much cityworks and economy issues to worry about. Not to mention, destroying everything in a city you are evetually going to conquer anyway hurts you more then the AI, since they get money assistance and you don't. Also, their government types get build immediately after conquest, unlike the player who has to wait for years to get good MIC buildings in place.