If my Sovereign is a Mage, I have him join up with me at first and maybe get him a level, possibly two. Then use him to manage unrest.
With everything else, I am always looking for a good a mage to run an army, and I might level him up more. But if he isnt suited for being a Mage, unrest management.
Most end up being weaker than a unit you can build, and trying to level them late game can end up troublesome.
Early game they eat way too much exp, and it simply isnt worth it.
All in all if they didnt manage unrest I probably would just dismiss them. (Unless I need a Mage, or they have magic spells I could use)