they can actually cause quite an amount of unrest - just test it - the seleukids have many spies in that area and when you have some good spy or assassin just kill one of them in your town an than you can see a difference in civil unrest, spies are able to even provoke revolts quite easy and early in this region - the problem is that you will have to oppose a stronger rebel garrison than factional garrison in some rare cases,
when you have a problem with capturing the easternmost settlements of AS - send spies, when you have problems with their public order, send a trained spy usually there is a Saka spy in one of them and you will have to throw him out or kill him in order to get control of the town (same with the AS southeastern provinces - often AS has 2 or more spies in or near them and they really suck)