I've find city states don't do much to help the war effort when I am allied to them and they don't do much to threaten me when they are allied against me. They just sit there with rare breakouts, even if they have weaponry superior to mine. For example, playing at prince as Askia as I go on my warring way, I leave my home territory little protected. Though there may be several city states there, they keep to themselves and never try to make an attempt on any of my cities or pillage any of my tiles (even if they are allied to enemies and militaristic) - though I may only have an archer or xbow in each city and no cav to hunt them down. What seems to be foolish risk taking on my part turns out to be no problem at all. If I were the AI, I would be continually sending enemy ally CS units on raids, knocking out vital mines and pastures - but they just sit there. When I am allied with a CS and fighting an enemy on its borders, the CS will then pitch in and give some help, sometimes, but still pretty bashfully. For combat, the single best use of CS's I have found is to ally with one or more that are deep in enemy territory then gift them heavy hitting units such as cannon/artillery to draw off and destroy enemy units that might otherwise come my way. Again, this shows their essentially defensive posture - they fight, but only if attacked.