I find an assaisan useless without a spy. I put a spy to find targets for the assaisan. And whenever the enemy recruit spies/assaisans/dplomats, I kill them. I learnt that leaving them is a threat in the hard way. I was Scythia and I was netural with Mecedonia after cracking up Thracians. In 1 turn they attacked the spy had opened the gates. Believe me, my infantry is baaaad to their Royals. So I died miserabely.