True that, but two fully held duchies will usually fill one's demesne limit anyway, so there's seldom much point to holding anything beyond that.A few comments:
There is a major exception to the rule of wanting to own every county in a duchy--namely, when the duchy is uncreated. Then you can safely give some counties to vassals without their desiring the duchy title.
I'm not sure the money you'd get from holding say, Byzantium would be worth it over holding two full duchies, as the duke would desire the county.Also, I think you should probably mention that sometimes certain provinces are so rich that they merit holding yourself, even if they are outside of your home duchies.
It's not as if your vassals can do all that much with money anyway. A single city vassal will not let them hire mercenaries, only possibly upgrade a building once in a while, or more likely: fail to assassinate someone.In the section on keeping vassals weak, you recommend giving out counties with all the lower titles included. This may be good advice for keeping them happy, but it is sub-optimal for keeping them weak. If you want to keep the new count weak, keep the one or more of the richest holdings (usually cities) as your direct holding/vassal. The relations hit for each holding you keep in your demesne/as a direct vassal is -25. This means it is not usually a problem in the first generation, since he is +40 grateful for having been made a count.
As such, I don't think it is worth it to have vassals in other noble's counties.