Originally posted by Park Ranger
This isn't the only thing I use spies for, but it is the most useful, IMO. And don't forget - you can't investigate cities with an embassy if you're at war - the embassy will be closed. With a spy, however, you can investigate a city any time. Useful at times if your rival is to strong to attack indiscriminately or if you want to track Wonder production.
I've been working under the assumption that as long as I can read details of enemy militaries on the Military Advisor screen, then my spy must be in place (ie not exposed by the enemy). It also seems that the MA is the only advisor who keeps track of what government each civ is using.