Everybody is "complaining" about the high upkeep of client rulers, but let us take a look at the benefits and compare the costs:
1. Client rulers got the finest troops as bodyguards, especially the Celtic Lesser King, and since your are likely to encounter some enemy/rebel activity in this province, you probably would need those soldiers at least once to fight. What would it cost to put a unit of Neitos to garrison a town?
2. Client rulers are not family members, but often got good traits to lead an army, or to just defend the town; plus they can hire mercs from those territories, so you can use those as quick response to enemy activities there, to hire them, upgrade and send towards front line, and with a merc captain ancillary you save 10% for the units recruitment - the less you pay the better, eh? ;)
3. Especially in provinces with mines you can boost up your income at least by 20% or even more. You can also bump the growth, the farming and trade income of the settlement; and the most important thing is you become a significant law bonus, even more once that guy becomes a good administrator, and he probably will since you will be producing quite a few troops from that town.
4. And last but not least, through all those bonuses mentioned above once the population "calms down", you won´t need a single unit to help maintaining public order, especially interesting for larger settlements, where you´d need a bunch of soldiers to simply keep the people from rioting, and this without any single bonus you´d get from the client ruler. And at the same time your FM can move on leading your armies, or govern already developed towns of your own.

I love those guys, the only think is, its sometimes a pity to get a great allied general, just to keep him in that damn small town, while most of your FM kind of suck...^^