Kings are good if you want to delegate authority. In other words, delegate land management, delegate minor wars, delegate vassal management. You'll get less personal taxes and levies, but your vassals will expand your empire and improve their holdings, if you let them (laws and such).
It does reduce the pool for councilors, but you can still look for tutors in your vassal's lands, just use the people search option instead of the diplomacy screen. A vassal of your vassal will always accept a contract to tutor someone in my experience (assuming they're not already tutoring 2 kids).
Currently, the only issue I find with king vassals is if they move their capital to a stupid place that makes getting their levies annoying ( like away from the coast in a coastal kingdom), or if their levies get so big that they attrition before you can get them into battle, but this is a reported bug that is slated for fixing. Also, if a king gains independence, inviting them back to the empire is either much harder or impossible. Kings in my game have the "I'm a king!" acceptance malus and it doesn't display any of the perks.