Don't you think that its weird that tribal vassals can choose which wars to fight, while their liege, an independent ruler, will be forced into all wars of their allies, no matter the circumstances, without any choice?
Horde vassals operate the same way as tribals (absent a blood pact with overlord) so it's not like they are some super-special exception. Then there's all the crappy stuff tribals have to deal with (fully empowered council, god-awful inheritance laws, no income to speak of, etc.). I think having one very slight advantage over feudal vassals as a counter-balance isn't the worst thing in the world...
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