I think early on its something that you shouldn't be worrying about, its an excess drain on income that you don't need and that you be far better spent on building up castle towns and other economic infrastructure. But after about 50 years I find that I have a lot of excess moneys due to high tax laws, lots of economic infrastructure and fighting people; claiming the gold for sieges etc. This means that when my character dies and the heir inherits I find that a good popularity boost is to create the duchies and give them to my new, young vassals. Later down the game they become more aggravating sure, but I think in the long term it helps. I mean prestige is important, the more your character has, the more your vassals will respect him, so swings and roundabouts really, but the prestige bonus snowballs and is a useful feature in vassal popularity management. Another instance where I would create a duchy title and give it away, would be when 1 of the leaders of a country in the de jury duchy is planning to rebel/hates me/ demanding lots of things in factions etc. I would not give it to this individual, but instead give it to another count who would gain his lands and so the event of independence for example in factions would be removed, or another similar event would disappear. This would only really appear early on in the game, when your at your most vulnerable.