Like Grimmy, I use champions for XP training and usually want better cultural conversion with dignataries. For generals, in the early to mid campaign I'm usually hoping to boost morale and combat stats (in late campaign, unit stats can be boosted by other methods such as military traditions, better-quality weapons and armour as well as improved technology).
I wonder if your strategies for generals and agents changes in different periods of the campaign? My empire's economy usually peaks in the mid-campaign (after that, enemies conquer more of my trade partners, other trade partners become hostile because of my expansion, corruption rises and I'm fielding more expensive high-tier units). Also, the 'cultural difference' penalty to public order has been reduced in some of my regions, where I've had temples (and sometimes dignitaries) for a long time.
This means that, in the late campaign, dignitaries become more useful attached to an army (to reduce unit upkeep) or even to increase the loyalty of a rival party (if the army is lead by a general belonging to a rival party and if the dignitary has the 'Advisor' skill). The dignitary skill tree (for Rome) is
here. Both 'Militarist' and 'Economist' skills reduce unit upkeep, I wonder if an agent can have both and if the reductions stack?
I wonder if there are other useful advantages (perhaps in specific cultures, as skills are culture-specific) which people are using, which it's easy to miss if you have a standard way to level up all agents of a specific type? For example, the Slaver skill for dignitaries gives the parent army up to 60% more battle captives - a huge bonus for anyone using enslaved enemies to boost your economy - with up to 30% higher morale for unarmoured infantry. That sounds as if it's designed for a specific faction or strategy which relies on unarmoured infantry who enslave their defeated foes, I wonder which faction(s) they had in mind.