I feel like I'm rescuiing caged prisoners just for the sake of it, but I don't really see a bonus anywhere (As in extra experience for instance), so do objectives like saving 8 prisoners grant any extra experience? If so, when? Because it doesn't happen immediatly at least. Does rescuiing prisoners after the 8th one grant any additional experience or is it just in vain?
Bonus objectives grant extra experience, and there's an achievement for rescuing all of the prisoners(there's more than 8), but I don't believe there are any other benefits besides these.
The achievement is "Hero of Alcarnus", it is listed under Challenge Achievements, not the general campaign achievements.
Additionally, rescued prisoners generally give you a random amount of gold.