Downside to the client rules should be noted:
- They cost a small fortune
- They do count against spawning new family members

Spies are very useful opening gates of enemy cities, and for watching enemy movement in border areas. They have great line of sight. They need to be trained though, and they are relatively useless performing operations, until they have gained 4 or 5 skill points. Same with assassins. Espionage on armies led by captains / assassinations on armies led by captains are usually your best bets to train these units up.

Not sure if BI really enhances the AI much. It is probably a bit faction dependent.