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I'm pretty sure that for some reason King's don't get claims on the de jure kingdom, only on de jure duchies that they own. I might have it wrong, though.
 
Now I see no reason to usurp a kingdom.
1. You gain prestige.
2. The previous kingdom fractures as all its duchies become independent.
3. Claimants to those duchies whose claims you press will become your vassals.
 
Now I see only one reason because:
1) You gain a lot more prestige holding a lot of duchies without being king
2) If you can usurp the title you are probably stronger than the king already

The third option is situational and not always justify to usurp it I think.
 
Now I see only one reason because:
1) You gain a lot more prestige holding a lot of duchies without being king
2) If you can usurp the title you are probably stronger than the king already

The third option is situational and not always justify to usurp it I think.
Well personally, number 3 is the one of the most useful ways to expand your kingdom when you have got to that level of power. There's also the fact that if you hold ridiculous numbers of duke titles then all of your vassals will hate you, which will screw up your tax income and make it hard to change laws.
 
For point 2, if you don't usurp the title you can only declare war once every 10 years without a prestige hit. If you usurp it they fracture and you can declare war on each duchy separately and take their land in a shorter amount of time.
 
I was holding 6 duchies before usurping scotland and had no relation penalty. Now I'm down to 2 and people don't like me as before, I earn the same amount of prestige, don't have any claimant on any independant duchy from de jure kingdom and no CB.
 
I fought a war to put my dad on the French throne, most of the vassals went to him after victory.