I think they are ok but I usually cut their number in half (12 on a huge map) as I found them impeding on my growth and the A.I growth as well.
But they are very exploitable. for example you can usually use city state as a training ground for my troops, and in a marathon lenght game this can be obscene advantage. You kill most of their troops, leave an archer in the city and let the city and the archer shoot at your troops while hammering them with ranged units and let them heal back, rince repeat. After awhile you have troops with over 300 xp